Update defaultContext.jsonld

defaultContext/defaultContext.jsonld

 {
   "@context": [
-    "https://fiware.github.io/NGSI-LD_Tests/ldContext/defaultContextVocab.jsonld",
+    "https://forge.etsi.org/gitlab/NGSI-LD/NGSI-LD/raw/master/defaultContext/defaultContextVocab.jsonld",
     "https://forge.etsi.org/gitlab/NGSI-LD/NGSI-LD/raw/master/coreContext/ngsi-ld-core-context.json"
   ]
 }