Fix build of documentation

Oversight in 1c42346, which has incorrectly shaped a <xref> markup.
Only v10 and v9.6 got broken.

Reported-by: Stefan Kaltenbrunner
Discussion: https://postgr.es/m/675cde90-a8dc-faeb-4701-d35a89ee06a2@kaltenbrunner.cc

doc/src/sgml/ref/pg_receivewal.sgml

@@ -54,11 +54,11 @@ PostgreSQL documentation
    The option <option>--synchronous</> must be specified to flush WAL data
    in real time. Since <application>pg_receivewal</application> does not
    apply WAL, you should not allow it to become a synchronous standby when
-   <xref linkend="guc-synchronous-commit"/> equals
+   <xref linkend="guc-synchronous-commit"> equals
    <literal>remote_apply</literal>. If it does, it will appear to be a
    standby that never catches up, and will cause transaction commits to
    block. To avoid this, you should either configure an appropriate value
-   for <xref linkend="guc-synchronous-standby-names"/>, or specify
+   for <xref linkend="guc-synchronous-standby-names">, or specify
    <varname>application_name</varname> for
    <application>pg_receivewal</application> that does not match it, or
    change the value of <varname>synchronous_commit</varname> to