Please Update the Documentation

Hey,

I like your product a lot!
Nevertheless, I the Documentation regard the ZeppelinContext and its Object Exchange

https://zeppelin.apache.org/docs/0.9.0-SNAPSHOT/usage/other_features/zeppelin_context.html#object-exchange

Needs to be updated.

It's not working to retrieve

val myPythonDataFrame = z.get("myPythonDataFrame").asInstanceOf[DataFrame]

If I remove `.asInstanceOf[DataFrame]`, then it's fine. Maybe I am missing some libs to import.

Another issue is

# Exchanging data frames
myPythonDataFrame = ...
z.put("myPythonDataFrame", postsDf._jdf)

It should be

# Exchanging data frames
myPythonDataFrame = ...
z.put("myPythonDataFrame", myPythonDataFrame )

Right?
Maybe I miss something and don't understand `._jdf`