fix errant verb tense in some messages if pet or mon name ends in 's'

Report stated:
"Poes deliberately slither onto a polymorph trap!" ... it's only one cat, er,
black naga. Why does the parser treat the name as plural? There are lots of
singular words and names that end in -s or -es!

H9249 1780

src/decl.c

@@ -253,7 +253,8 @@ struct c_common_strings c_common_strings = { "Nothing happens.",
                                              "You can move again.",
                                              "Never mind.",
                                              "vision quickly clears.",
-                                             { "the", "your" } };
+                                             { "the", "your" },
+                                             { "mon", "you" } };
 
 /* NOTE: the order of these words exactly corresponds to the
    order of oc_material values #define'd in objclass.h. */