fix github issue #1413 - mimic feedback for gold

Issue reported by ars3niy:  if a mimic was given the shape of a
gold piece it gets reported as 2 gold pieces but the message was
|A gold pieces appears next to you.

Avoid article "A" prefix, and use plural verb "appear" instead of
singular "appears", yielding
|Gold pieces appear next to you.

Fixes #1413

src/pager.c

@@ -220,7 +220,7 @@ mhidden_description(
             what = (otmp && otmp->otyp != STRANGE_OBJECT)
                    ? simpleonames(otmp)
                    : obj_descr[STRANGE_OBJECT].oc_name;
-            if (incl_article)
+            if (incl_article && (!otmp || otmp->quan == 1L))
                 what = an(what);
             Strcat(outbuf, what);