Update the datastore documentation regarding the usage of st2kv

[cyberkov]

Hello!

I came across an error in the documentation on how to use the decryption of items in the st2kv.

I just replaced the occurences of "jinja filter" with "argument" of which I am not entirely sure is the correct way to describe the feature. Please rephrase my bad english as you see fit ;-)

Cheers Hannes