Change escape-time to 50

I don't know why I had set it to 0 before, but using oh-my-zsh to
auto-start tmux with an escape time of 0 causes it to spit out some
ANSI shit every time. Changing it to 50 fixes that.

See tmux-plugins/tmux-sensible#61

configs/tmux.conf

@@ -1,5 +1,5 @@
 set-option -g prefix C-a
-set -s escape-time 0
+set -s escape-time 50
 
 set-window-option -g xterm-keys on
 set -g update-environment -r