DOC: 1.1.5 release date (#38318)

Author: simonjayhawkins

doc/source/whatsnew/v1.1.5.rst

@@ -1,7 +1,7 @@
 .. _whatsnew_115:
 
-What's new in 1.1.5 (??)
-------------------------
+What's new in 1.1.5 (December 07, 2020)
+---------------------------------------
 
 These are the changes in pandas 1.1.5. See :ref:`release` for a full changelog
 including other versions of pandas.