Add quoting for sample command

Add quoting for sample command

The user's guide talks about renaming workspaces, for example to
"2: mail", and a sample key binding for use with i3-input is supplied.
However, this example lacks proper quoting for the format string, so
that when workspace name with a space in it, like "2: mail", is given,
the current workspace is renamed to "2:". This patch adds proper
quoting.