Update instructions for downloading the certificate

[datanerd]

This pull request updates the documentation for enabling Microsoft Entra SAML SSO with GitHub Enterprise Managed User. The main change clarifies the instructions for downloading the SAML certificate and updates the screenshot reference to match the new instructions.

Documentation improvements:

• Updated the instructions to download the Base64 certificate from the SAML Certificates section, replacing the previous reference to the SAML Signing Certificate section and the PEM certificate.
• Changed the screenshot image reference to common/certificate-base64.png to reflect the updated certificate download process.

[learn-build-service-prod]

Learn Build status updates of commit 570cb59:

💡 Validation status: suggestions

File	Status	Preview URL	Details
docs/identity/saas-apps/common/certificate-base64.png	💡Suggestion		Details
docs/identity/saas-apps/github-enterprise-managed-user-tutorial.md	✅Succeeded

docs/identity/saas-apps/common/certificate-base64.png

• Line 0, Column 0: [Suggestion: image-name-incomplete - See documentation] The image name contains an incomplete word, misspelled word, acronym, or abbreviation that is disallowed.

For more details, please refer to the build report.

Note: Your PR may contain errors or warnings or suggestions unrelated to the files you changed. This happens when external dependencies like GitHub alias, Microsoft alias, cross repo links are updated. Please use these instructions to resolve them.

[datanerd]

@microsoft-github-policy-service agree

[prmerger-automator]

@datanerd : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[ttorble]

@nguhiu

Can you review the proposed changes? Note that, before we can merge the PR, the thumbprint, notification email, and login url values in the image must be obscured by using a color block that matches the background. Thanks.

IMPORTANT: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team

[learn-build-service-prod]

Learn Build status updates of commit 7aa73fd:

💡 Validation status: suggestions

File	Status	Preview URL	Details
docs/identity/saas-apps/common/certificate-base64.png	💡Suggestion		Details
docs/identity/saas-apps/github-enterprise-managed-user-tutorial.md	✅Succeeded

docs/identity/saas-apps/common/certificate-base64.png

• Line 0, Column 0: [Suggestion: image-name-incomplete - See documentation] The image name contains an incomplete word, misspelled word, acronym, or abbreviation that is disallowed.

For more details, please refer to the build report.

Note: Your PR may contain errors or warnings or suggestions unrelated to the files you changed. This happens when external dependencies like GitHub alias, Microsoft alias, cross repo links are updated. Please use these instructions to resolve them.

[prmerger-automator]

PRMerger Results

Issue	Description
Added File(s)	This PR contains added files. New files require human review.
Image File	This PR added or updated an image file(s).

[datanerd]

I've blurred out the thumbnail and reuploaded the image