README Doc: Link to Airflow directory in ASF Directory (#11137)

`https://downloads.apache.org` -> `https://downloads.apache.org/airflow` (links to precise dir)

(cherry picked from commit af3c677)

README.md

@@ -120,7 +120,7 @@ Apache Airflow is an [Apache Software Foundation](http://www.apache.org) (ASF) p
 and our official source code releases:
 
 - Follow the [ASF Release Policy](http://www.apache.org/legal/release-policy.html)
-- Can be downloaded from [the ASF Distribution Directory](https://downloads.apache.org)
+- Can be downloaded from [the ASF Distribution Directory](https://downloads.apache.org/airflow)
 - Are cryptographically signed by the release manager
 - Are officially voted on by the PMC members during the
   [Release Approval Process](http://www.apache.org/legal/release-policy.html#release-approval)