Fix incorrect parameter name in a function header comment

Author: Zhijie Hou
Discussion: https://postgr.es/m/14cd74ea00204cc8a7ea5d738ac82cd1@G08CNEXMBPEKD05.g08.fujitsu.local
Backpatch-through: 12, where the mistake was introduced

src/backend/utils/cache/lsyscache.c

@@ -874,7 +874,7 @@ get_attnum(Oid relid, const char *attname)
 /*
  * get_attgenerated
  *
- *		Given the relation id and the attribute name,
+ *		Given the relation id and the attribute number,
  *		return the "attgenerated" field from the attribute relation.
  *
  *		Errors if not found.