Fixed the version in the changelog

Author: agronholm

docs/versionhistory.rst

@@ -4,7 +4,7 @@ Version history
 To find out how to migrate your application from a previous version of
 APScheduler, see the :doc:`migration section <migration>`.
 
-**v4.0.0a4**
+**4.0.0a4**
 
 - **BREAKING** Renamed any leftover fields named ``executor`` to ``job_executor``
   (this breaks data store compatibility)