doc: Fix purported type of pg_am.amhandler to match reality.

Joel Jacobson

doc/src/sgml/catalogs.sgml

@@ -571,7 +571,7 @@
 
      <row>
       <entry><structfield>amhandler</structfield></entry>
-      <entry><type>oid</type></entry>
+      <entry><type>regproc</type></entry>
       <entry><literal><link linkend="catalog-pg-proc"><structname>pg_proc</structname></link>.oid</literal></entry>
       <entry>
        OID of a handler function that is responsible for supplying information