Update gui_mac.txt headings

Update gui_mac.txt headings to match MacVim release.

Author: dkav

runtime/doc/gui_mac.txt

@@ -1,10 +1,8 @@
-*gui_mac.txt*	For Vim version 8.2.  Last change: 2020 Aug 13
+*gui_mac.txt*	For MacVim Release 176.  Last change: 2023 Feb 7
 
+			 MacVim - help file
 
-		  VIM REFERENCE MANUAL    by Bjorn Winckler
-
-
-The MacVim Graphical User Interface			*macvim* *gui-macvim*
+The MacVim Graphical User Interface		*macvim* *gui-macvim*
 
  1. MacVim differences		|macvim-differences|
  2. Starting MacVim		|macvim-start|