fix off-by-one error (closes #21330)

Author: benjaminp

Doc/howto/unicode.rst

@@ -49,7 +49,7 @@ another and managed to catch on.
 
 255 characters aren't very many.  For example, you can't fit both the accented
 characters used in Western Europe and the Cyrillic alphabet used for Russian
-into the 128--255 range because there are more than 127 such characters.
+into the 128--255 range because there are more than 128 such characters.
 
 You could write files using different codes (all your Russian files in a coding
 system called KOI8, all your French files in a different coding system called