DOC: fixes Sphinx parallel build error on doc/source/whatsnew/v0.24.0.rst (#54935)

Add missing StringIO import

Co-authored-by: johanna.trost <[email protected]>

Author: JohannaTrost

doc/source/whatsnew/v0.24.0.rst

@@ -286,6 +286,7 @@ value. (:issue:`17054`)
 
 .. ipython:: python
 
+    from io import StringIO
     result = pd.read_html(StringIO("""
       <table>
         <thead>