[gdb/tui] Fix help text of show tui tab-width

I noticed:
...
(gdb) help show tui tab-width
Show the tab witdh, in characters, for the TUI.
This variable controls how many spaces are used to display a tab character.
...
a typo: "witdh".

Fix this by using "width" instead.

Reviewed-By: Tom Tromey <tom@tromey.com>

gdb/tui/tui-win.c

diff --git a/gdb/tui/tui-win.c b/gdb/tui/tui-win.c
index 7abd1e2..08a21fb 100644
--- a/gdb/tui/tui-win.c
+++ b/gdb/tui/tui-win.c
@@ -1255,7 +1255,7 @@ This variable controls the attributes to use for the active window border:\n\
   add_setshow_zuinteger_cmd ("tab-width", no_class,
 			     &internal_tab_width, _("\
 Set the tab width, in characters, for the TUI."), _("\
-Show the tab witdh, in characters, for the TUI."), _("\
+Show the tab width, in characters, for the TUI."), _("\
 This variable controls how many spaces are used to display a tab character."),
 			     tui_set_tab_width, tui_show_tab_width,
 			     &tui_setlist, &tui_showlist);