docs(ingest) Add MetadataChangeProposalWrapper import to example code

[iprentic]

The code doesn't run as is, need this import as well. Also, change the initialized emitter to the rest emitter to localhost, for continuity with the above curl command, and an example that can run out of the box.

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable). If a new feature has been added a Usage Guide has been added for the same.
• For any breaking change/potential downtime/deprecation/big changes an entry has been made in Updating DataHub

[hsheth2]

Thanks!

[hsheth2]

@yoonhyejin - lower priority, but it'd be great to move these embedded code blocks into standalone files that are inlined into the docs. That way our linter would catch these sorts of issues in the future