change is_greystone() to is_graystone()

-Rename is_greystone() to is_graystone() since I've
had one complaint about my choice of spelling for
the macro already.

-Change the recent "#rub touchstone" code to use
the macro which pre-existed under the other spelling
and was already used in the very same "if" statement
with that spelling in invent.c. :-)

include/obj.h

@@ -217,7 +217,7 @@ struct obj {
 #define MAX_OIL_IN_FLASK 400	/* maximum amount of oil in a potion of oil */
 
 /* special stones */
-#define is_greystone(obj)	((obj)->otyp == LUCKSTONE || \
+#define is_graystone(obj)	((obj)->otyp == LUCKSTONE || \
 				 (obj)->otyp == LOADSTONE || \
 				 (obj)->otyp == FLINT     || \
 				 (obj)->otyp == TOUCHSTONE)