docs: fix image in README.md (#1068)

Author: bshaffer

README.md

@@ -1,6 +1,6 @@
 oauth2-server-php
 =================
 
-[![Total Downloads](https://poser.pugx.org/bshaffer/oauth2-server-php/downloads.png)](https://packagist.org/packages/bshaffer/oauth2-server-php)
+[![Total Downloads](https://poser.pugx.org/bshaffer/oauth2-server-php/downloads)](https://packagist.org/packages/bshaffer/oauth2-server-php)
 
 View the [complete documentation](https://bshaffer.github.io/oauth2-server-php-docs/)