doc: Update URL for plr

Author: petere

doc/src/sgml/external-projects.sgml

@@ -190,7 +190,7 @@
      <row>
       <entry>PL/R</entry>
       <entry>R</entry>
-      <entry><ulink url="http://www.joeconway.com/plr/">http://www.joeconway.com/plr/</ulink></entry>
+      <entry><ulink url="http://www.joeconway.com/plr.html">http://www.joeconway.com/plr.html</ulink></entry>
      </row>
 
      <row>