doc: fix two id/xreflabel inconsistencies in config.sgml

Hi hackers,

Attached is a tiny docs patch that fixes two SGML attribute
inconsistencies in config.sgml.

1. The varlistentry for enable_group_by_reordering has
id="guc-enable-groupby-reordering" — every other enable_*
parameter in the file uses the guc-enable-X-y-z form that
mirrors the underscores in the GUC name. Renaming to
guc-enable-group-by-reordering matches that convention.

2. The varlistentry for quote_all_identifiers has
xreflabel="quote-all-identifiers" with hyphens — every other
xreflabel in the file matches the GUC name verbatim with
underscores. Fixing to xreflabel="quote_all_identifiers".

Neither id has any incoming linkend reference, so this is purely a
consistency change with no callsite churn.

I used an LLM (Claude Code) as part of preparing this — it spotted
the inconsistencies during a sweep of config.sgml. I reviewed the
diff myself before sending.

Regards,
Bill Kim