fix main search shortcut typo

The main menu was typo-ed (see below), due to the change in 
https://github.com/djcb/mu/commit/957c5ceaef3a9542a14a93051bd8493ff4b8638a

  Basics

	* [j]ump to some maildir
	* enter a [/]earch query
	* [C]ompose a new message

Since the bookmark key itself didn't change from "s", this seems like a typo.