Add expires example to proof.

index.html

@@ -935,6 +935,27 @@ <h3>Proofs</h3>
 representing time values to individuals.
         </p>
 
+        <pre class="example nohighlight"
+        title="A proof attached to a data document that uses the 'expires' property">
+{
+  "@context": [
+    {"myWebsite": "https://vocabulary.example/myWebsite"},
+    "https://w3id.org/security/data-integrity/v2"
+  ],
+  "myWebsite": "https://hello.world.example/",
+  "proof": {
+    "type": "DataIntegrityProof",
+    "cryptosuite": "ecdsa-rdfc-2019",
+    "created": "2020-06-11T19:14:04Z",
+    <span class="comment">// the proof expires a month after it was issued</span>
+    <span class="highlight">expires": "2020-07-11T19:14:04Z"</span>,
+    "verificationMethod": "https://ldi.example/issuer#zDnaepBuvsQ8cpsWrVK<wbr>w8fbpGpvPeNSjVPTWoq6cRqaYzBKVP",
+    "proofPurpose": "assertionMethod",
+    "proofValue": "z98X7RLrkjnXEADJNUhiTEdwyE5GXX8cyJZRLQZ7vZyUXb23Zkdakf<wbr>RJ7adYY8hn35EetqBkNw813SGsJHWrcpo4"
+  }
+}
+        </pre>
+
       <p>
 The Data Integrity specification supports the concept of multiple
 proofs in a single document. There are two types of multi-proof