Fix for old pandas

Author: HyukjinKwon

databricks/koalas/frame.py

@@ -1819,7 +1819,7 @@ def apply(self, func, axis=0):
             ... def length(s) -> int:
             ...    return len(s)
             ...
-            >>> df = ks.DataFrame(range(1000), columns=['A'])
+            >>> df = ks.DataFrame({'A': range(1000)})
             >>> df.apply(length, axis=0)  # doctest: +SKIP
             0     83
             1     83