Update link for pllua

Author: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/A05874AE-8771-4C61-A24E-0B6249B8F3C2@yesql.se

doc/src/sgml/external-projects.sgml

@@ -199,7 +199,7 @@
      <row>
       <entry>PL/Lua</entry>
       <entry>Lua</entry>
-      <entry><ulink url="https://github.com/pllua/pllua"></ulink></entry>
+      <entry><ulink url="https://github.com/pllua/pllua-ng"></ulink></entry>
      </row>
 
      <row>