doc: fix pg_get_multixact_stats() num_mxids type (integer -> bigint)

Hi hackers,

func-info.sgml documents the num_mxids column of
pg_get_multixact_stats() as integer, but pg_proc.dat declares it int8
(bigint), and has since the function was added (97b101776ce). The
attached one-line patch fixes the doc.

The same doc text is reported in REL_19_STABLE, so this likely needs a
backpatch to 19 (I only checked master).

Regards,
Naeem Akhter
Percona