Fix version in subtitle.

Author: Omer Katz

docs/getting-started/introduction.rst

@@ -39,7 +39,7 @@ What do I need?
 ===============
 
 .. sidebar:: Version Requirements
-    :subtitle: Celery version 5.0 runs on
+    :subtitle: Celery version 5.1 runs on
 
     - Python ❨3.6, 3.7, 3.8❩
     - PyPy3.6 ❨7.3❩