storage(docs): storage.bucket -> storage().bucket

Update example from docs to show:

    storage().bucket('name');

instead of:

    storage.bucket('name');

lib/storage/index.js

@@ -80,7 +80,7 @@ var STORAGE_BASE_URL = 'https://www.googleapis.com/storage/v1/b';
  * //-
  *
  * // Access `storage` through the `gcloud` module directly.
- * var musicBucket = gcloud.storage.bucket('music');
+ * var musicBucket = gcloud.storage().bucket('music');
  *
  * //-
  * // Elsewhere.
@@ -92,8 +92,8 @@ var STORAGE_BASE_URL = 'https://www.googleapis.com/storage/v1/b';
  *   projectId: 'my-project'
  * });
  *
- * var albums = myProject.storage.bucket('albums');
- * var photos = myProject.storage.bucket('photos');
+ * var albums = myProject.storage().bucket('albums');
+ * var photos = myProject.storage().bucket('photos');
  *
  *
  * // Override default configuration details.
@@ -137,8 +137,8 @@ function Storage(config) {
  *   keyFilename: '/path/to/keyfile.json'
  * });
  *
- * var albums = gcloud.storage.bucket('albums');
- * var photos = gcloud.storage.bucket('photos');
+ * var albums = gcloud.storage().bucket('albums');
+ * var photos = gcloud.storage().bucket('photos');
  */
 Storage.prototype.bucket = function(options) {
   if (util.is(options, 'string')) {