Fix CalVer badge.

Author: jezdez

README.rst

@@ -22,7 +22,7 @@ The documentation can be found on `python-dockerflow.readthedocs.io`_
    :alt: Documentation Status
    :target: https://python-dockerflow.readthedocs.io/en/latest/?badge=latest
 
-.. image:: https://img.shields.io/badge/calver-YY.MM.PATCH-22bfda.svg
+.. image:: https://img.shields.io/badge/calver-YYYY.M.PATCH-22bfda.svg
    :target: https://calver.org/
    :alt: CalVer - Timely Software Versioning