Fix typo in `Delta_For_Thresholding.pdf` link URL.
Fix typo in `Delta_For_Thresholding.pdf` link URL.

This link works: https://github.com/google/differential-privacy/blob/main/common_docs/Delta_For_Thresholding.pdf
The previous link doesn't: https://github.com/google/differential-privacy/blob/main/common_docs/.Delta_For_Thresholding.pdf

PiperOrigin-RevId: 433027845

tensorflow/federated,tensorflow/federated,tensorflow/federated