chore: update codecov badge

Signed-off-by: Brian Faust <[email protected]>

Author: Brian Faust

README.md

@@ -7,7 +7,7 @@
 > A simple PHP Cryptography Implementation for the Ark Blockchain.
 
 [![Build Status](https://travis-ci.org/ArkEcosystem/php-crypto.svg)](https://travis-ci.org/ArkEcosystem/php-crypto)
-[![Codecov](https://img.shields.io/codecov/c/github/arkecosystem/php-crypto.svg)](https://codecov.io/gh/arkecosystem/php-crypto)
+[![Codecov](https://img.shields.io/codecov/c/github/arkecosystem/php-crypto/master.svg)](https://codecov.io/gh/arkecosystem/php-crypto)
 [![Latest Version](https://img.shields.io/github/release/ArkEcosystem/php-crypto.svg)](https://github.com/ArkEcosystem/php-crypto/releases)
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)