[gdb/tui] Revert workaround in tui_source_window::show_line_number

The m_digits member of tui_source_window is documented as having semantics:
...
  /* How many digits to use when formatting the line number.  This
     includes the trailing space.  */
...

The commit 1b6d4bb2232 ("Redraw both spaces between line numbers and source
code") started printing two trailing spaces instead:
...
-  xsnprintf (text, sizeof (text), "%*d ", m_digits - 1, lineno);
+  xsnprintf (text, sizeof (text), "%*d  ", m_digits - 1, lineno);
...

Now that PR30325 is fixed, this no longer has any effect.

Fix this by reverting to the original behaviour: print one trailing space
char.

Tested on x86_64-linux.

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