Update ldap.md If you escape the '\' character when using a "domain\user" scheme for the bind_dn name, it will fail. gitlab-ctl reconfigure escapes characters, and if you pre-escape the '\', the resultant yaml file will have four (4) '\' characters

What does this MR do?

Correct the example for users with a "domain\username" bind_dn username.

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Why was this MR needed?

If the existing example is followed, the resultant yaml file will not work.

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Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

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