documentation typo in ALTER TABLE example

failed: regress/postgres_fdw, regress/prepared_xacts, regress/brin, regress/gin, regress/gist, regress/spgist, regress/privileges, regress/init_privs, regress/security_label, regress/collate, regress/matview, regress/lock, regress/replica_identity, regress/rowsecurity, regress/password, regress/object_address, regress/tablesample, regress/groupingsets, regress/drop_operator, regress/large_object, regress/alter_generic, regress/alter_operator, regress/misc, regress/psql, regress/async, regress/dbsize, regress/misc_functions, regress/sysviews, regress/tsrf, regress/tidscan, regress/stats_ext, regress/rules, regress/psql_crosstab, regress/select_parallel, regress/publication, regress/subscription, regress/amutils, regress/select_views, regress/portals_p2, regress/foreign_key, regress/cluster, regress/dependency, regress/guc, regress/bitmapops, regress/combocid, regress/tsearch, regress/tsdicts, regress/foreign_data, regress/window, regress/xmlmap, regress/functional_deps, regress/advisory_lock, regress/json, regress/jsonb, regress/json_encoding, regress/indirect_toast, regress/equivclass, regress/plancache, regress/limit, regress/plpgsql, regress/copy2, regress/temp, regress/domain, regress/rangefuncs, regress/prepare, regress/without_oid, regress/conversion, regress/truncate, regress/alter_table, regress/sequence, regress/identity, regress/polymorphism, regress/rowtypes, regress/returning, regress/largeobject, regress/with, regress/xml, regress/event_trigger, regress/stats