Fix typo in src/backend/utils/cache/lsyscache.c

Hi

I found the comment of function get_attgenerated(Oid relid, AttrNumber attnum) seems wrong.
It seems the function is given the attribute number not the name.

/*
  * 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.

Best regards,
houzj