Update v0.21.0.txt

Author: Giftlin

doc/source/whatsnew/v0.21.0.txt

@@ -566,7 +566,7 @@ Categorical
 - Bug in the categorical constructor with empty values and categories causing
   the ``.categories`` to be an empty ``Float64Index`` rather than an empty
   ``Index`` with object dtype (:issue:`17248`)
-- Bug in categorical operations `Series.cat <categorical.cat>' not preserving the original Series' name (:issue:`17509`)
+- Bug in categorical operations :ref:`Series.cat <categorical.cat>' not preserving the original Series' name (:issue:`17509`)
 
 PyPy
 ^^^^