docs: Fix verb form 'be' to 'been' in about_tmux.md

Author: tony

docs/about_tmux.md

@@ -257,7 +257,7 @@ This could be because:
 - tmux server has killed its last session, killing the server.
 - tmux server has encountered a crash. (tmux is highly stable,
   this will rarely happen)
-- tmux has not be launched yet at all.
+- tmux has not been launched yet at all.
 
 :::