Add backticks in documentation

Author: ri938

doc/source/whatsnew/v0.20.3.txt

@@ -37,7 +37,7 @@ Performance Improvements
 Bug Fixes
 ~~~~~~~~~
 - Fixed issue with dataframe scatter plot for categorical data that reports incorrect column key not found when categorical data is used for plotting (:issue:`16199`)
-- Bug in reindexing on an empty CategoricalIndex (:issue:`16770`)
+- Bug in reindexing on an empty ``CategoricalIndex`` (:issue:`16770`)