Update usage for file name

What does this MR do?

Updates the usage of 'file name' (two words) to 'filename' one word, as per the word list.

Changes to usage in GraphQL docs will be in a separate MR.

Related issues

Author's checklist

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

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• If the content requires it, ensure the information is reviewed by a subject matter expert.
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