docs(about_tmux): Fix orphan footnote reference

Change [1] to [^id2] to properly reference the defined footnote for
the tmux manpage source link.

Author: tony

docs/about_tmux.md

@@ -572,7 +572,7 @@ line. The interval is determined by the `status-interval`:
 | `C-Up, C-Down` `C-Left, C-Right` | Resize the current pane in steps of one cell.                                                                              |
 | `M-Up, M-Down` `M-Left, M-Right` | Resize the current pane in steps of five cells.                                                                            |
 
-Source: tmux manpage[1].
+Source: tmux manpage[^id2].
 
 To get the text documentation of a `.1` manual file: