Fix teacher email's label to say student's grade

[Imran92]

Proposed Changes

• When a student completes a course, we send an email to the teacher. Now it says "Your grade" in it. I should be "Student's grade"

Testing Instructions

• Go to Sensei LMS -> Tools -> Click on "Run Action" of the "Recreate Emails" tool
• Go to Sensei LMS -> Settings -> Emails (tab) -> Teacher Emails (tab)
• Open the "Course Completed" Email and make sure the grade label is "Student's Grade"
• Now take a course as a student and complete it
• Teacher's received email should have the new label

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Legacy courses (course without blocks) are tested
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[donnapep]

Made it even simpler. 🙂