Fix JSON literal example and update Gemfile.

Author: gkellogg

Gemfile.lock

@@ -1,6 +1,6 @@
 GIT
   remote: git://github.com/ruby-rdf/json-ld.git
-  revision: 2a11745bc50eab0d5a4c5c3a5cd98b840176896e
+  revision: 6f4b20de173ef27ff3959f5206b9a7cb813ecde7
   branch: develop
   specs:
     json-ld (3.0.2)

index.html

@@ -4796,7 +4796,7 @@ <h2>Data Round Tripping</h2>
            data-to-rdf>
       <!--
       @prefix ex: <http://example.org/vocab#> .
-      @prefix jsonld: <http://www.w3.org/ns/json-ld#> .
+      @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
       [ex:c14n ****"""[56,{"1":[],"10":null,"d":true}]"""^^rdf:JSON****] .
       -->
       </pre>