Update tutorial failure after GPyTorch update

Summary:
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

Author: Carl Hvarfner