Use commit url for @openzeppelin/test-environment example (#551)

Author: frangio

README.md

@@ -211,5 +211,5 @@ $ yarn
 [32]: https://github.com/sc-forks/buidler-e2e/tree/coverage
 [33]: https://github.com/sc-forks/moloch
 [34]: https://github.com/sc-forks/solidity-coverage/blob/master/docs/advanced.md#reducing-the-instrumentation-footprint
-[35]: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/scripts/coverage.js
+[35]: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/e5fbbda9bac49039847a7ed20c1d966766ecc64a/scripts/coverage.js