chore(tmux): escape backslash for tmux 3
chore(tmux): escape backslash for tmux 3

One of the breaking changes in v3:

> INCOMPATIBLE: tmux's configuration parsing has changed to use yacc(1).
> There is one incompatible change: a \ on its own must be escaped or
> quoted as either \\ or '\' (the latter works on older tmux versions).

I originally made this change in my dotfiles here:

  https://github.com/lencioni/dotfiles/commit/369fdf588309366f37d58411226ebbded8b2fe41

It looks like this could also be quoted so it continues to work on
older versions

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent