Fix invalid CSS cursor value for .phabricator-main-menu-brand

Summary:
The W3C CSS validator throws Value Error : cursor hand is not a cursor value : hand.
hand is not a valid value for cursor; likely pointer was meant.
See https://developer.mozilla.org/en-US/docs/Web/CSS/cursor

Test Plan: Paste main-menu-view.css into https://jigsaw.w3.org/css-validator/ and read some CSS docs.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25922