Fixed typo in uplift_trees_with_synthetic_data.ipynb

[jroessler]

Proposed changes

Fixed typo (See #529 )
from 'http://proceedings.mlr.press/v67/gutierrez17a/gutierrez17a.pdfc' to 'http://proceedings.mlr.press/v67/gutierrez17a/gutierrez17a.pdf'

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[jeongyoonlee]

Thanksm @jroessler for catching it!