Update pubspec.yaml to 2.0.1 (#615)

Author: jjliu15

packages/google_mobile_ads/pubspec.yaml

@@ -13,7 +13,7 @@
 #  limitations under the License.
 
 name: google_mobile_ads
-version: 2.0.0
+version: 2.0.1
 description: Flutter plugin for Google Mobile Ads, supporting
   banner, interstitial (full-screen),  rewarded and native ads
 repository: https://github.com/googleads/googleads-mobile-flutter/tree/main/packages/google_mobile_ads