runtime(doc): using wrong highlight for UTF-8

This happens, because UTF-8 was on a single line and as such triggered
the helpHeading syntax group. So slightly re-worded the line, so
that it does no longer match the header syntax rule.

fixes: #15278

Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: chrisbra

runtime/doc/mbyte.txt

@@ -1,4 +1,4 @@
-*mbyte.txt*     For Vim version 9.1.  Last change: 2024 Jun 09
+*mbyte.txt*     For Vim version 9.1.  Last change: 2024 Jul 17
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar et al.
@@ -1379,7 +1379,7 @@ Motif.  Use the ":hi Menu font={fontname}" command for this. |:highlight|
 TYPING UTF-8						*utf-8-typing*
 
 If you are using X-Windows, you should find an input method that supports
-UTF-8.
+the UTF-8 encoding.
 
 If your system does not provide support for typing UTF-8, you can use the
 'keymap' feature.  This allows writing a keymap file, which defines a UTF-8