DOC: Fix sphinx warning (#18741)

[ci skip]

It interpreted that as a link.

Author: TomAugspurger

doc/source/whatsnew/v0.21.1.txt

@@ -123,7 +123,7 @@ I/O
 - Bug in :func:`read_csv` when reading numeric category fields with high cardinality (:issue:`18186`)
 - Bug in :meth:`DataFrame.to_csv` when the table had ``MultiIndex`` columns, and a list of strings was passed in for ``header`` (:issue:`5539`)
 - Bug in parsing integer datetime-like columns with specified format in ``read_sql`` (:issue:`17855`).
-- Bug in :meth:`DataFrame.to_msgpack` when serializing data of the numpy.bool_ datatype (:issue:`18390`)
+- Bug in :meth:`DataFrame.to_msgpack` when serializing data of the ``numpy.bool_`` datatype (:issue:`18390`)
 - Bug in :func:`read_json` not decoding when reading line deliminted JSON from S3 (:issue:`17200`)
 - Bug in :func:`pandas.io.json.json_normalize` to avoid modification of ``meta`` (:issue:`18610`)
 - Bug in :func:`to_latex` where repeated multi-index values were not printed even though a higher level index differed from the previous row (:issue:`14484`)