update whatsnew entry

Author: ruiann

doc/source/whatsnew/v0.21.0.txt

@@ -704,6 +704,7 @@ Groupby/Resample/Rolling
 - Bug in ``DataFrame.groupby`` where index and column keys were not recognized correctly when the number of keys equaled the number of elements on the groupby axis (:issue:`16859`)
 - Bug in ``groupby.nunique()`` with ``TimeGrouper`` which cannot handle ``NaT`` correctly (:issue:`17575`)
 - Bug in ``DataFrame.groupby`` where a single level selection from a ``MultiIndex`` unexpectedly sorts (:issue:`17537`)
+- Bug in ``TimeGrouper`` differs when passes as a list and as a scalar (:issue:`17530`)
 
 Sparse
 ^^^^^^