Make extension capability example easier to understand

The other example contained some misleading suggestions,
such as including authentication tokens for intermediary
nodes in the body of the capabilities.

Closes #819

Author: shs96c

webdriver-spec.html

@@ -1723,30 +1723,12 @@ <h2>Protocol Extensions</h2>
  <pre>{
   "capabilities": {
     "alwaysMatch": {
-      // a cloud provider’s authentication token
-      "sauce:token": "b448b871-a526-4634-aefc-26df6eede519",
-
-      // an <a>intermediary node</a>’s access details
-      "intermediary:login": {
-        "username": "maja",
-        "password": "donut"
-      }
-    },
-    "firstMatch": [
-      {
-        // browser specific configuration
-        "moz:firefoxOptions": {
-          "binary": "/usr/bin/firefox",
-          "args": ["--no-remote"],
-          "prefs": {
-            "dom.ipc.processCount": "8"
-          },
-          "log": {
-            "level": "trace"
-          }
-        }
+      // browser specific configuration
+      "&lt;prefix&gt;:browserOptions": {
+        "binary": "/usr/bin/browser-binary",
+        "args": ["--start-page=http://example.com"],
       }
-    ]
+    }
   }
 }</pre>
 </aside>