Update release date for 5.1.0b4

Author: Jibola

docs/source/releases/5.1.x.rst

@@ -5,7 +5,7 @@ Django MongoDB Backend 5.1.x
 5.1.0 beta 4
 ============
 
-*August 12, 2025*
+*August 13, 2025*
 
 - Backward-incompatible: Removed support for database caching as the MongoDB security
   team considers the cache backend's ``pickle`` encoding of cached values a