Change Airflow version to 2.0.0a1 in Updating.md (#11508)

UPDATING.md

@@ -25,7 +25,7 @@ assists users migrating to a new version.
 <!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
 **Table of contents**
 
-- [Airflow Master](#airflow-master)
+- [Airflow 2.0.0a1](#airflow-200a1)
 - [Airflow 1.10.13](#airflow-11013)
 - [Airflow 1.10.12](#airflow-11012)
 - [Airflow 1.10.11](#airflow-11011)
@@ -47,7 +47,7 @@ assists users migrating to a new version.
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
 
-## Airflow Master
+## Airflow 2.0.0a1
 
 The 2.0 release of the Airflow is a significant upgrade, and includes substantial major changes,
 and some of them may be breaking. Existing code written for earlier versions of this project will may require updates