Expand sample keymaps to ameliorate OSX features

OS X, for reasons of its own, swallows the CTRL+O keystroke, so OS X
users are likely to end up looking at this file to figure out how to fix
their keymap. This change adds an example of the mapping they're looking
for.

The secondary reason for documenting this is that none of the examples
show a multi-word name, so it took me a while to figure out the correct
name to use here. Adding an example makes it clearer how to use
multi-word names.

Change-Id: I7b95eea720864a5b18b9b5796bc30b4aebf57248

examples/reference-gertty.yaml

@@ -97,6 +97,8 @@ keymaps:
     diff: 'd'
   - name: custom
     review: ['r', 'R']
+  - name: osx #OS X blocks ctrl+o
+    change-search: 'ctrl s'
 
 # The default keymap may be selected with the '-k KEYMAP' command line
 # option, or with the following line: