Minor document typo

Hi,

doc/src/sgml/monitoring.sgml seems to have a minor typo:

In pg_stat_database_conflicts section (around line 3621) we have:

      <para>
       Number of uses of logical slots in this database that have been
       canceled due to old snapshots or too low a <xref linkend="guc-wal-level"/>
       on the primary
      </para></entry>

I think "too low a" should be "too low" ('a' is not
necessary). Attached is the patch.

Best reagards,
--
Tatsuo Ishii
SRA OSS LLC
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp