doc: Fix grammar in page of pg_receivewal

Introduced by f61e1dd.

Author: Kyotaro Horiguchi
Discussion: https://postgr.es/m/20211026.112304.1962954080884317968.horikyota.ntt@gmail.com

Author: michaelpq

doc/src/sgml/ref/pg_receivewal.sgml

@@ -90,7 +90,7 @@ PostgreSQL documentation
 
     <listitem>
      <para>
-      If a starting point cannot not be calculated with the previous method,
+      If a starting point cannot be calculated with the previous method,
       and if a replication slot is used, an extra
       <command>READ_REPLICATION_SLOT</command> command is issued to retrieve
       the slot's <literal>restart_lsn</literal> to use as starting point.