doc: Fix description about pg_stat_statements.track_planning.

This commit fixes wrong wording like "a fewer kinds"
in the description about track_planning option.

Back-patch to v13 where pg_stat_statements.track_planning was added.

Author: Justin Pryzby
Reviewed-by: Julien Rouhaud, Fujii Masao
Discussion: https://postgr.es/m/20210418233615.GB7256@telsasoft.com

Author: MasaoFujii

doc/src/sgml/pgstatstatements.sgml

@@ -696,8 +696,9 @@
       <varname>pg_stat_statements.track_planning</varname> controls whether
       planning operations and duration are tracked by the module.
       Enabling this parameter may incur a noticeable performance penalty,
-      especially when a fewer kinds of queries are executed on many
-      concurrent connections.
+      especially when statements with identical query structure are executed
+      by many concurrent connections which compete to update a small number of
+      <structname>pg_stat_statements</structname> entries.
       The default value is <literal>off</literal>.
       Only superusers can change this setting.
      </para>