Administration: Fix typo in `forms.css` after [46866].

A stray `e` made browsers ignore a CSS ruleset preventing select elements from changing color on hover.

Props ryokuhi.
See #48420.
Fixes #50580.

Built from https://develop.svn.wordpress.org/trunk@48362


git-svn-id: https://core.svn.wordpress.org/trunk@48131 1a063a9b-81f0-0310-95a4-ce76da25c4cd