WORKSPACE file link broken

## Issue Description
Execute the "bazel build -c opt //..." would faile.Because the link in the WORKSPACE file have been broken, and the
the download of dependent third-party compressed package will failed.
Type: *bug report* or *feature request*

### Describe what happened (or what feature you want)

### Describe what you expected to happen

### How to reproduce it (as minimally and precisely as possible)

1. cd sentinel-cpp
2. run the “bazel build -c opt //...”

### Tell us your environment
Ubuntu18.04

### Anything else we need to know?

@xcaspar @BlackImpl @dapingzhang @x-cold

Start by opening the WORKSPACE file and reproducing the failure with `bazel build -c opt //...` from the sentinel-cpp repository on Ubuntu 18.04. Identify the broken third-party package link and update it so dependency downloads complete and the full Bazel build succeeds.

Written by the indexing model from the issue text.