Fix typo for log comments (#437)

Summary:
Pull Request resolved: #437

Only comment typo fixing.

Reviewed By: vancexu

Differential Revision: D37793695

fbshipit-source-id: 0b90138422ad89ac87be080d7da329e9adeb2b43

Author: Yishuo Wang

torcharrow/idataframe.py

@@ -274,7 +274,7 @@ def log(self) -> DataFrame:
         >>> df = ta.dataframe({"a": [1,2,None,4],
                                 "b": [5, 6, None, 8]
                                 })
-        >>> df.min()
+        >>> df.log()
         index         a        b
         -------  --------  -------
             0  0         1.60944