fixed a doc string format

Author: smartvinnetou

pandas/core/generic.py

@@ -7134,11 +7134,11 @@ def asof(self, where, subset=None):
         --------
         Show which entries in a DataFrame are NA.
 
-        >>> df = pd.DataFrame({{'age': [5, 6, np.NaN],
-        ...                    'born': [pd.NaT, pd.Timestamp('1939-05-27'),
+        >>> df = pd.DataFrame(dict(age=[5, 6, np.NaN],
+        ...                    born=[pd.NaT, pd.Timestamp('1939-05-27'),
         ...                             pd.Timestamp('1940-04-25')],
-        ...                    'name': ['Alfred', 'Batman', ''],
-        ...                    'toy': [None, 'Batmobile', 'Joker']}})
+        ...                    name=['Alfred', 'Batman', ''],
+        ...                    toy=[None, 'Batmobile', 'Joker']))
         >>> df
            age       born    name        toy
         0  5.0        NaT  Alfred       None