Clean up some documentation for log_statement_sample_rate

This was missing from 88bdbd3f.

Author: Christoph Berg, Justin Pryzby
Discussion: https://postgr.es/m/20190403215938.GA26375@alvherre.pgsql
Discussion: https://postgr.es/m/20190330224333.GQ5815@telsasoft.com

doc/src/sgml/config.sgml

@@ -5887,7 +5887,7 @@ local0.*    /var/log/postgresql
         <para>
          Determines the fraction of statements that exceed
          <xref linkend="guc-log-min-duration-statement"/> to be logged.
-         The default is <literal>1</literal>, meaning log all such
+         The default is <literal>1.0</literal>, meaning log all such
          statements.
          Setting this to zero disables logging by duration, same as setting
          <varname>log_min_duration_statement</varname> to