Bump 5.1.0b2 release date (#291)

Author: timgraham

docs/source/releases/5.1.x.rst

@@ -5,7 +5,7 @@ Django MongoDB Backend 5.1.x
 5.1.0 beta 2
 ============
 
-*April 18, 2025*
+*April 21, 2025*
 
 - Backward-incompatible: :class:`~django_mongodb_backend.fields.ArrayField`\'s
   :attr:`~.ArrayField.size` parameter is renamed to