Fix local exchangeId in exchange example.

As this example is a single exchange, the exchange id should be the same in both participate calls.
See issue w3c-ccg#425 for discussion.

index.html

@@ -1214,7 +1214,7 @@ <h4>Exchange Examples</h4>
     autonumber
     Note right of H: Start exchange
     W->>I: Initiate
-    Note right of W: POST /workflows/123/exchanges/123 &mdash; HTTP request to start exchange (e.g., send credentials, get credentials)
+    Note right of W: POST /workflows/123/exchanges/abc &mdash; HTTP request to start exchange (e.g., send credentials, get credentials)
     I->>W: Verifiable Presentation Request (VPR)
     Note left of I: VPR includes method of interaction, for purposes of exchange
     W->>I: Verifiable Presentation (VP)