add whatsnew for the fix for pandas-dev#17407

Author: huashuai

doc/source/whatsnew/v0.21.0.txt

@@ -517,6 +517,7 @@ Indexing
 - Bug in ``CategoricalIndex`` reindexing in which specified indices containing duplicates were not being respected (:issue:`17323`)
 - Bug in intersection of ``RangeIndex`` with negative step (:issue:`17296`)
 - Bug in ``IntervalIndex`` where performing a scalar lookup fails for included right endpoints of non-overlapping monotonic decreasing indexes (:issue:`16417`, :issue:`17271`)
+- Bug in ``Series.renam`` when called with `str` will alter name of series rather than index of series. (:issue:`17407`)
 
 I/O
 ^^^