Doc: fix a few oddly-spelled SGML ID attributes.

Doc: fix a few oddly-spelled SGML ID attributes.

Avoid use of "_" in SGML IDs.  Awhile back that was actually
disallowed by the toolchain, as a consequence of which our convention
has been to use "-" instead.  Fix a couple of stragglers that are
particularly inconsistent with that convention and with related IDs.

This is just neatnik-ism, so no need for back-patch.

Discussion: https://postgr.es/m/769446.1673478332@sss.pgh.pa.us