Fix invalid CSS property value

Summary: x is not a valid unit.

Test Plan: Open a Maniphest task in the web browser; inspect the CSS of the label of any items in the "Add Action" dropdown.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25510