Fix single "Objective-C" typo in developer facing documentation

[d-ronnqvist]

Bug/issue #, if applicable: rdar://154206938

Summary

This fixes a single "Objective C" → "Objective-C" typo in the developer facing documentation for the @AlternateRepresentation directive.

Dependencies

None

Testing

• Build and preview the documentation
• Visit the local documentation for the @AlternateRepresentation directive
  • The 2nd paragraph of the discussion should end with

    [...], or the NS_SWIFT_NAME macro in Objective-C.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• [ ] Added tests n/a
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary