Fix call to undefined method PhutilJSON::encodeAsObject()

Summary:
Replace call to undefined method PhutilJSON::encodeAsObject() with existing PhutilJSON::encodeFormatted()

EXCEPTION: (Error) Call to undefined method PhutilJSON::encodeAsObject() at [<phabricator>/src/applications/transactions/xaction/PhabricatorEditEngineDefaultTransaction.php:68]

Closes T15603

Test Plan: Grep and read the code in https://we.phorge.it/source/arcanist/browse/master/src/parser/PhutilJSON.php

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15603

Differential Revision: https://we.phorge.it/D25391