txnbuild/change_trust: change trust bug fix

[adirutwn]

If you're making a doc PR or something tiny where the below is irrelevant, just delete this
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PR Structure

• [✅] This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• [✅ ] This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• [✅ ] This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot

Thoroughness

• [✅ ] This PR adds tests for the most critical parts of the new functionality or fixes.

Summary

Make the BuildXDR() in txnbuild/change_trust return correct operation

Summary of changes

Change the return variable from xdr.Operation{Body: Body} to op

What shouldn't be reviewed

Unit test