Fix formatting of a keyboard shortcut in the README.md

This will format the `z` keyboard shortcut reference in this text in the
same way as all other keyboard shortcuts in the README.md file.

Author: DanOpcode

README.md

@@ -194,7 +194,7 @@ Say you're on a feature branch that was itself branched off of the develop branc
 
 ### Undo
 
-You can undo the last action by pressing 'z' and redo with `ctrl+z`. Here we drop a couple of commits and then undo the actions.
+You can undo the last action by pressing `z` and redo with `ctrl+z`. Here we drop a couple of commits and then undo the actions.
 Undo uses the reflog which is specific to commits and branches so we can't undo changes to the working tree or stash.
 
 [More info](/docs/Undoing.md)