docs: align error message punctuation to Terraform best practices

[lechnerc77]

Purpose

• Alignment of error messages according to Hashicorp best practices (see https://developer.hashicorp.com/terraform/plugin/framework/diagnostics#creating-diagnostics)

Does this introduce a breaking change?

[ ] Yes
[X] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[ ] Feature
[ ] Refactoring (no functional changes, no api changes)
[X] Documentation content changes
[ ] Other... Please describe:

How to Test

n/a

What to Check

Verify that the following are valid:

n/a

Other Information

n/a

Checklist for reviewer

The following organizational tasks must be completed before merging this PR:

• The PR is assigned to the Terraform project and a status is set (typically "in review").
• The PR has the matching labels assigned to it.
• The PR has a milestone assigned to it.
• If the PR closes an issue, the issue is referenced.
• Possible follow-up items are created and linked.

[rui1610]

Thanks. Looks good.
And I love it how little things like good documentation and proper error messages give our Terraform provider a professional "look and feel". Thanks again for keeping an eye on the quality of our documentation.