Correct fzf-tmux tmux checking bug

bin/fzf-tmux

@@ -82,7 +82,7 @@ while [ $# -gt 0 ]; do
   shift
 done
 
-if [ -z "$TMUX_PANE" ]; then
+if [ -z "$TMUX" ]; then
   fzf "${args[@]}"
   exit $?
 fi