fixed typo.

sql/item_strfunc.h

@@ -901,7 +901,7 @@ public:
   const char *func_name() const { return "collate"; }
   enum Functype functype() const { return COLLATE_FUNC; }
   virtual void print(String *str, enum_query_type query_type);
-  Item_field *filed_for_view_update()
+  Item_field *field_for_view_update()
   {
     /* this function is transparent for view updating */
     return args[0]->field_for_view_update();