Use latest javadoc URL for rdf4j

Author: acoburn

pom.xml

@@ -220,6 +220,7 @@
               <link>https://jena.apache.org/documentation/javadoc/jena/</link>
               <link>https://jena.apache.org/documentation/javadoc/arq/</link>
               <link>https://www.antlr.org/api/Java/</link>
+              <link>https://rdf4j.org/javadoc/5.1.0/</link>
               <link>http://www.javadoc.io/doc/com.fasterxml.jackson.core/jackson-databind/</link>
             </links>
             <header>