Fixed broken link to php mongodb installation in README.md

Author: lucho2d7

README.md

@@ -21,7 +21,7 @@ Table of contents
 Installation
 ------------
 
-Make sure you have the MongoDB PHP driver installed. You can find installation instructions at http://php.net/manual/en/mongodb.installation.php.php
+Make sure you have the MongoDB PHP driver installed. You can find installation instructions at http://php.net/manual/en/mongodb.installation.php
 
 **WARNING**: The old mongo PHP driver is not supported anymore in versions >= 3.0.