backticks in whatsnew doc

Author: erbian

doc/source/whatsnew/v0.20.3.txt

@@ -82,7 +82,7 @@ Sparse
 
 Reshaping
 ^^^^^^^^^
-- PeriodIndex / TimedeltaIndex.join was missing the sort= kwarg (:issue: `16541`) 
+- ``PeriodIndex`` / ``TimedeltaIndex.join`` was missing the ``sort=`` kwarg (:issue:`16541`) 
 - Bug in joining on a ``MultiIndex`` with a ``category`` dtype for a level (:issue:`16627`).
 - Bug in :func:`merge` when merging/joining with multiple categorical columns (:issue:`16767`)