Fix typo in pg_validatebackup documentation.

Author: Fujii Masao
Reviewed-by: Robert Haas
Discussion: https://postgr.es/m/78f76a3d-1a28-a97d-0394-5c96985dd1c0@oss.nttdata.com

doc/src/sgml/ref/pg_validatebackup.sgml

@@ -132,7 +132,7 @@ PostgreSQL documentation
       <listitem>
        <para>
         Exit as soon as a problem with the backup is detected. If this option
-        is not specified, <literal>pg_basebackup</literal> will continue
+        is not specified, <literal>pg_validatebackup</literal> will continue
         checking the backup even after a problem has been detected, and will
         report all problems detected as errors.
        </para>