Fixed definition of "timingProviderSource" property

The type of object returned by the property was wrong (TimingProviderObject
instead of simply TimingProvider).

Also the spec did not have text to explain what the property returned.

I'm not sure that the property is needed in practice, but it's harmless as
well...

index.html

@@ -580,7 +580,7 @@ <h2>Timing Object</h2>
            optional unrestricted double endPosition),
          Constructor (TimingProvider provider)]
         interface TimingObject : EventTarget {
-          readonly    attribute TimingProviderObject timingProviderSource;
+          readonly    attribute TimingProvider timingProviderSource;
           readonly    attribute TimingObjectState readyState;
           readonly    attribute unrestricted double startPosition;
           readonly    attribute unrestricted double endPosition;
@@ -598,7 +598,10 @@ <h2>Timing Object</h2>
           The <dfn>startPosition</dfn> and <dfn>endPosition</dfn> attributes MUST respectively be set to the <a>start position</a> and <a>end position</a> that the <a>state vector</a> may take, if any, or <code>-Infinity</code> and <code>Infinity</code> otherwise.
         </p>
         <p>
-          The <dfn>readyState</dfn> attribute MUST be set to the current state of the <a>timing provider object</a>. If the <a>timing object</a> represents an <a>internal timing resource</a>, this will typically always be <code>connecting</code> or <code>open</code>. The <a>readyState</a> attribute may take other values when the <a>timing object</a> represents an <a>external timing resource</a>.
+          The <dfn>readyState</dfn> attribute MUST be set to the current state of the <a>timing object</a>. If the <a>timing object</a> represents an <a>internal timing resource</a>, this will typically always be <code>connecting</code> or <code>open</code>. The <a>readyState</a> attribute may take other values when the <a>timing object</a> represents an <a>external timing resource</a>.
+        </p>
+        <p>
+          The <dfn>timingProviderSource</dfn> attribute MUST be set to the <a>timing provider source</a> of the <a>timing object</a>, if any, or null otherwise.
         </p>
       </div>