minor error in tutorial

[laurinehu]

Hi,

fyi, I noticed a small mistake in your tutorial ("RST-DT"/"trees and annotations" section). The indexing to recover the subtree ex_doc[2][0][0][1] is incorrect (index out of range). The correct indexing to get the subtree displayed as example seems to be is "[2][1][0][0]".

Anyway thank you for the useful tuto :)