fix #H4457 - grammar nitpick

"Your pair of speed boots glow silver for a moment." should be
"Your pair of speed boots glows silver for a moment.".  The fix
reverses a post-3.6.0 change to is_plural().  Also, add new
pair_of() to test for object formatted as "pair of Bars".  For verb
usage, that's definitely singular, but for pronoun usage, sometimes
plural seems better (although it might actually be incorrect).

I fixed up the formatting of a block comment in obj.h, but it is
still a candidate for tab cleanup.

src/invent.c

@@ -1489,7 +1489,7 @@ struct obj *otmp;
     }
     if (s1)
         pline("Use the '%s' command to %s %s%s.", s1, s2,
-              !is_plural(otmp) ? "that" : "those", s3);
+              !(is_plural(otmp) || pair_of(otmp)) ? "that" : "those", s3);
     else
 #endif
         pline(silly_thing_to, word);