Correct the URL pointing to PL/R

As pointed out by documentation comment, the URL for PL/R
needs to be updated to the correct current repository. Back-patch
to all supported branches.

doc/src/sgml/external-projects.sgml

@@ -205,7 +205,7 @@
      <row>
       <entry>PL/R</entry>
       <entry>R</entry>
-      <entry><ulink url="http://www.joeconway.com/plr.html"></ulink></entry>
+      <entry><ulink url="https://github.com/postgres-plr/plr"></ulink></entry>
      </row>
 
      <row>