DOC: Update release date for 2.1.2 (#55665)

Author: lithomas1

doc/source/whatsnew/v2.1.2.rst

@@ -1,6 +1,6 @@
 .. _whatsnew_212:
 
-What's new in 2.1.2 (October ??, 2023)
+What's new in 2.1.2 (October 25, 2023)
 ---------------------------------------
 
 These are the changes in pandas 2.1.2. See :ref:`release` for a full changelog