docs: Validation error with xmllint 2.15.0

I just upgraded to libxml2 2.15.0 and now the docs no longer validate
with the following error when running make html:

	/usr/bin/xmllint --nonet --path . --path . --output postgres-full.xml --noent --valid postgres.sgml
	ref/pg_combinebackup.sgml:317: validity error : standalone: normalization of attribute linkend on xref by external subset declaration
	   state of the cluster using <xref linkend="app-pgchecksums "/> is
	                                                              ^

Downgrading to libxml2 2.14.6 fixes the validation.  There's also an
unreleased fix [1] for this regression.  But I figure that it's best to
just remove the insignificant whitespace with the attached patch.

[1] https://gitlab.gnome.org/GNOME/libxml2/-/commit/da45a190f718e8e2f0e3d2a6325ffa23abc8b90c

-- 
Erik Wienhold