Backport PR #36887: DOC: 1.1.3 release date (#36891)

Co-authored-by: Simon Hawkins <[email protected]>

Author: meeseeksmachine

doc/source/whatsnew/v1.1.3.rst

@@ -1,7 +1,7 @@
 .. _whatsnew_113:
 
-What's new in 1.1.3 (??)
-------------------------
+What's new in 1.1.3 (October 5, 2020)
+-------------------------------------
 
 These are the changes in pandas 1.1.3. See :ref:`release` for a full changelog
 including other versions of pandas.