Fix typo at calendar transaction

Summary:

• Fixed conveted => converted

Ref T11576

Test Plan: * Looked at a page, where somebody converted an AllDay Event to a normal one

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Tags: #calendar

Maniphest Tasks: T11576

Differential Revision: https://secure.phabricator.com/D16488