[css-font-loading] Fix IDL error, .loaded is a Promise<FontFace>.

--HG--
extra : rebase_source : d4ee87487dac154522a8f16327c4011f5d655ea6

Author: tabatkins

css-font-loading/Overview.bs

@@ -101,7 +101,7 @@ The <code>FontFace</code> Interface</h2>
 			readonly attribute FontFaceLoadStatus status;
 
 			Promise&lt;FontFace> load();
-			readonly attribute Promise&lt;boolean> loaded;
+			readonly attribute Promise&lt;FontFace> loaded;
 		};
 	</pre>