DOC: Fix typo in example (pandas-dev#19537)

Fix typo in the example for pandas.io.formats.style.Styler.format

Author: ppflrs

pandas/io/formats/style.py

@@ -364,7 +364,7 @@ def format(self, formatter, subset=None):
         >>> df = pd.DataFrame(np.random.randn(4, 2), columns=['a', 'b'])
         >>> df.style.format("{:.2%}")
         >>> df['c'] = ['a', 'b', 'c', 'd']
-        >>> df.style.format({'C': str.upper})
+        >>> df.style.format({'c': str.upper})
         """
         if subset is None:
             row_locs = range(len(self.data))