Update tutorial failure after GPyTorch update (meta-pytorch#3188)

Summary:
Pull Request resolved: meta-pytorch#3188

One-line change to state_dict reating in notebook,
from `"_transformed_loc"` to  f"{`BUFFERED_PREFIX}loc`"
in the tutorial to accomodate upstream gpytorch change: cornellius-gp/gpytorch#2691

Reviewed By: Balandat

Differential Revision: D93032203

fbshipit-source-id: 5577c5200aea32e82355361fb3de08fe7ef6eced

Author: Carl Hvarfner