Small corrections to GraphQL style guide

What does this MR do?

Some small corrections to https://docs.gitlab.com/ee/development/api_graphql_styleguide.html

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 this is just a docsfix.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

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• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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