doc: Correct the description about the length of pg_stat_activity.query.

pg_stat_activity.query text is truncated at 1024 bytes. But previously
the document described that it's truncated at 1024 characters.
This was not accurate when considering multibyte characters.

Back-patch to v10 where this inaccurate description was added.

Author: Atsushi Torikoshi
Reviewed-by: Daniel Gustafsson, Fujii Masao
Discussion: https://postgr.es/m/cd5b49a5a14e887542f5f569c1c6bde2@oss.nttdata.com

doc/src/sgml/monitoring.sgml

@@ -894,7 +894,7 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
        <structfield>state</structfield> is <literal>active</literal> this field shows the
        currently executing query. In all other states, it shows the last query
        that was executed. By default the query text is truncated at 1024
-       characters; this value can be changed via the parameter
+       bytes; this value can be changed via the parameter
        <xref linkend="guc-track-activity-query-size"/>.
       </para></entry>
      </row>