docs: in mapping SQL to C data types, timestamp isn't a pointer

It is an int64.

Reported-by: ajulien@shaktiware.fr

Discussion: https://postgr.es/m/159845038271.24995.15682121015698255155@wrigleys.postgresql.org

Backpatch-through: 9.5

doc/src/sgml/xfunc.sgml

@@ -2226,7 +2226,7 @@ memcpy(destination->data, buffer, 40);
         </row>
         <row>
          <entry><type>timestamp</type></entry>
-         <entry><type>Timestamp*</type></entry>
+         <entry><type>Timestamp</type></entry>
          <entry><filename>datatype/timestamp.h</filename></entry>
         </row>
         <row>