Fix incorrect context URL in examples.

index.html

@@ -3665,7 +3665,7 @@ <h3>Zero-Knowledge Proofs</h3>
              title="Verifiable credential using the Data Integrity BBS Cryptosuite with a Base Proof">
 {
   "@context": [
-    "https://www.w3.org/2018/credentials/v2",
+    "https://www.w3.org/ns/credentials/v2",
     "https://w3id.org/citizenship/v3"
   ],
   "type": ["VerifiableCredential", "PermanentResidentCard"],
@@ -3711,11 +3711,11 @@ <h3>Zero-Knowledge Proofs</h3>
         <pre class="example nohighlight"
              title="Verifiable presentation using the Data Integrity BBS Cryptosuite with a derived credential and proof">
 {
-  @context: "https://www.w3.org/2018/credentials/v2"
+  @context: "https://www.w3.org/ns/credentials/v2"
   type: "VerifiablePresentation",
   verifiableCredential: {
     "@context": [
-      "https://www.w3.org/2018/credentials/v2",
+      "https://www.w3.org/ns/credentials/v2",
       "https://w3id.org/citizenship/v3"
     ],
     "type": ["VerifiableCredential", "PermanentResidentCard"],
@@ -4470,7 +4470,7 @@ <h3>Media Type Precision</h3>
             </li>
             <li>
 Ensure that the first element of the `@context` property matches
-`https://www.w3.org/2018/credentials/v2`.
+`https://www.w3.org/ns/credentials/v2`.
             </li>
             <li>
 Assume an `application/vp` media type if the JSON document contains a top-level