runtime(doc): rename last t_BG reference to t_RB

This was missed in 37c64c7.

closes: #16491

Signed-off-by: Jon Parise <jon@indelible.org>
Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: jparise

runtime/doc/options.txt

@@ -1,4 +1,4 @@
-*options.txt*	For Vim version 9.1.  Last change: 2025 Jan 14
+*options.txt*	For Vim version 9.1.  Last change: 2025 Jan 20
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -1033,7 +1033,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	When the |t_RB| option is set, Vim will use it to request the background
 	color from the terminal.  If the returned RGB value is dark/light and
 	'background' is not dark/light, 'background' will be set and the
-	screen is redrawn.  This may have side effects, make t_BG empty in
+	screen is redrawn.  This may have side effects, make |t_RB| empty in
 	your .vimrc if you suspect this problem.  The response to |t_RB| can
 	be found in |v:termrbgresp|.