Fix typo in comments

Hi,

When reviewing other patches, I noticed two typos:

1.
src/backend/parser/gram.y
ALTER TABLE <name> ALTER [COLUMN] <colname> RESET ( column_parameter = value [, ... ] )

RESET cannot specify value.

2.
src/backend/utils/adt/xid8funcs.c
* Same as pg_current_xact_if_assigned() but doesn't assign a new xid if there

pg_current_xact_if_assigned() should be pg_current_xact_id()

Best regards,
Hou zhijie