doc: Remove stale entry for removed aclitem[] ~ aclitem operator

Hi,

Commit 2f70fdb0644 removed the operator ~(aclitem[],aclitem), but
missed removing its entry from the aclitem operators table in the
documentation.

When running the following SQL:

postgres=# SELECT oid::regoperator, oprcode FROM pg_operator WHERE
oprright = 'aclitem'::regtype;
oid | oprcode
-----------------------+-------------
+(aclitem[],aclitem) | aclinsert
-(aclitem[],aclitem) | aclremove
@>(aclitem[],aclitem) | aclcontains
=(aclitem,aclitem) | aclitemeq
(4 rows)

But, ~(aclitem[],aclitem) is documented.

This patch removes it. Thoughts?

--
Best regards,
Shinya Kato
NTT OSS Center