[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>