Better type definition

Fixes the `Invalid type(s) for operator IN.` warnings from the closure compiler.

src/ol/format/WMSCapabilities.js

@@ -446,8 +446,7 @@ function readCapabilityLayer(node, objectStack) {
  * @return {Object|undefined} Layer object.
  */
 function readLayer(node, objectStack) {
-  const parentLayerObject = /**  @type {Object.<string,*>} */
-        (objectStack[objectStack.length - 1]);
+  const parentLayerObject = /**  @type {!Object.<string,*>} */ (objectStack[objectStack.length - 1]);
 
   const layerObject = pushParseAndPop({}, LAYER_PARSERS, node, objectStack);