Add an optional step that describes how to change the
keyboard map in single user mode.

Differential Revision:	    https://reviews.freebsd.org/D890
Reviewed by:		    wblock


git-svn-id: svn+ssh://svn.freebsd.org/doc/head@45719 c2e8774f-c49f-e111-b436-862b2bbc8956