Customize: Use typographic quotation marks in the strings added in [41768].

Props audrasjb, tobifjellner.
Fixes #42290. See #40104.
Built from https://develop.svn.wordpress.org/trunk@41956


git-svn-id: https://core.svn.wordpress.org/trunk@41790 1a063a9b-81f0-0310-95a4-ce76da25c4cd