Fix capitalization of "merge requests" in notifications

What does this MR do?

As part of #233449 (closed) - let's fix the bad capitalization of "merge requests" in our notifications:

• Only capitalize the "M" if "merge" is the first word in the sentence or the slug
• Never capitalize the "R"

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• 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)

spec/mailers/emails/merge_requests_spec.rb

@@ -31,7 +31,7 @@
       aggregate_failures do
         is_expected.to have_referable_subject(merge_request, reply: true)
         is_expected.to have_body_text(project_merge_request_path(project, merge_request))
-        is_expected.to have_body_text('You have been mentioned in Merge Request')
+        is_expected.to have_body_text('You have been mentioned in merge request')
         is_expected.to have_link(merge_request.to_reference, href: project_merge_request_url(merge_request.target_project, merge_request))
         is_expected.to have_text_part_content(assignee.name)
         is_expected.to have_text_part_content(reviewer.name)