DOC: Fixes flake8 issues in whatsnew v0.13.* pandas-dev#24256

Added missing space in setup.cfg exclusion section.

Author: Seuss27

setup.cfg

@@ -45,6 +45,7 @@ ignore = E402,  # module level import not at top of file
          E703,  # statement ends with a semicolon
 
 exclude =
+
     doc/source/whatsnew/v0.15.0.rst
     doc/source/whatsnew/v0.15.1.rst
     doc/source/whatsnew/v0.15.2.rst