Update ngsi-ld-core-context.json to include ngsi-ld prefix

coreContext/ngsi-ld-core-context.json

 {
     "@context": {
+      "ngsi-ld": "http://uri.etsi.org/ngsi-ld/",    
       "id": "@id",
       "type": "@type",
       "value": "http://uri.etsi.org/ngsi-ld/hasValue",