Fix mistake in disconnect timestamp field name

This was correct (matching the format) in the types and in existing
usage elsewhere, but wrong in the actual data attached to events, so
this optional field was effectively never set.

Author: pimterry

src/server/mockttp-server.ts

@@ -1284,7 +1284,7 @@ ${await this.suggestRule(request)}`
                     ...eventData,
                     timingEvents: {
                         ...eventData.timingEvents,
-                        disconnectedTimestamp: now()
+                        disconnectTimestamp: now()
                     }
                 });
             });