#enhance docs again

Since "weapons and spell skills" is grammatically suspect and
"weapons and spells skills" sounds odd, change to "weapon and spell skills"
in both the Guidebook and the game's online descriptions.  The #enhance
command itself uses "current skills" and doesn't need any alteration.

src/cmd.c

@@ -1548,7 +1548,7 @@ struct ext_func_tab extcmdlist[] = {
 	{"conduct", "list voluntary challenges you have maintained",
 						doconduct, TRUE},
 	{"dip", "dip an object into something", dodip, FALSE},
-	{"enhance", "advance or check weapons and spell skills",
+	{"enhance", "advance or check weapon and spell skills",
 						enhance_weapon_skill, TRUE},
 	{"force", "force a lock", doforce, FALSE},
 	{"invoke", "invoke an object's powers", doinvoke, TRUE},