update what's new with bug fix

Author: sdementen

doc/source/whatsnew/v0.20.0.txt

@@ -416,6 +416,7 @@ Performance Improvements
 Bug Fixes
 ~~~~~~~~~
 
+- Bug in ``Timestamp.replace`` wrong exception raised when wrong argument name given (:issue:`15240`)
 - Bug in ``Index`` power operations with reversed operands (:issue:`14973`)
 - Bug in ``TimedeltaIndex`` addition where overflow was being allowed without error (:issue:`14816`)
 - Bug in ``TimedeltaIndex`` raising a ``ValueError`` when boolean indexing with ``loc`` (:issue:`14946`)