Fix incorrect comment.

src/termhooks.h

@@ -288,12 +288,12 @@ enum {
   /* The next four modifier bits are used also in keyboard events at
      the Lisp level.
 
-     It's probably not the greatest idea to use the 2^23 bit for any
+     It's probably not the greatest idea to use the 2^28 bit for any
      modifier.  It may or may not be the sign bit, depending on
-     VALBITS, so using it to represent a modifier key means that
+     FIXNUM_BITS, so using it to represent a modifier key means that
      characters thus modified have different integer equivalents
      depending on the architecture they're running on.  Oh, and
-     applying XINT to a character whose 2^23 bit is set sign-extends
+     applying XINT to a character whose 2^28 bit is set might sign-extend
      it, so you get a bunch of bits in the mask you didn't want.
 
      The CHAR_ macros are defined in lisp.h.  */