Fix code comments with bad capitalization

What does this MR do?

Fixes to code comments in the app, to correct the capitalization of "merge requests."

Related to #233449 (closed) where I'm collecting all the effort.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• Documentation (if required)
• Code review guidelines
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• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #233449 (closed)