Point the URL to PL/py directly to the page about the procedural language.

It used to point to a top-level page that contains client-side tools as
well. It was hard to find the procedural language there.

doc/src/sgml/external-projects.sgml

@@ -197,7 +197,7 @@
      <row>
       <entry>PL/Py</entry>
       <entry>Python</entry>
-      <entry><ulink url="http://python.projects.postgresql.org/">http://python.projects.postgresql.org/</ulink></entry>
+      <entry><ulink url="http://python.projects.postgresql.org/backend/">http://python.projects.postgresql.org/backend/</ulink></entry>
      </row>
 
      <row>