Fix markdown compile warning from stray chars

What does this MR do?

As part of the Kramdown fixes in !141317 (merged) I opted to NOT fix this warning, because I knew it would require a long pipeline to fix:

kramdown warning(s) for <Nanoc::Core::CompilationItemRepView item.identifier=/ee/api/graphql/reference/index.md name=default>
  Found no end tag for 'Other' (line 6808) - auto-closing it
  Found no end tag for 'Other' (line 1) - auto-closing it

(doc/api/graphql/reference/index.md is compiled from GraphQL definitions, which meant the problem was in the backend, rather than in doc/)

It's a small fix. The <Other projects> string in line 28 of ee/app/graphql/mutations/ci/runners/export_usage.rb should be in backticks, rather than straight quotes, so the parser won't try to do anything with it.

Related issues

Related to Resolve various Kramdown warnings (!141317 - merged) where I fixed the Kramdown warnings that did NOT require the long backend pipeline.

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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Reviewer's checklist

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