doc: updated how to unset an environment variable in tmux

docs/tmux.md

@@ -1,21 +1,25 @@
 # `tmux`
+
 ## Exporting a Variable
+
 [tmux is exporting an environment variable that is no longer being exported in .bashrc](https://superuser.com/questions/1216492/tmux-is-exporting-an-environment-variable-that-is-no-longer-being-exported-in-b)
-```
-tmux set-environment -r FZF_TMUX_OPTS
+
+```sh
+tmux set-environment -gru FZF_TMUX_OPTS
 ```
 
 ## Rename a pane
-```
+
+```tmux
 set -g pane-border-status top
 set -g pane-border-format " [ ###P #T ] "
 ```
-```
+
+```tmux
 CTRL+b + :
 select-pane -T "title"
 ```
 
-
 ## Sharing a Window Across Sessions
 
 To share a window between two sessions: `tmux link-window -s <src-window> -t <dst-window>`