Update reference to HTTPS

README.md

@@ -1,7 +1,7 @@
 # PSR Http Message Util
 
 This repository holds utility classes and constants to facilitate common
-operations of [PSR-7](http://www.php-fig.org/psr/psr-7/); the primary purpose is
+operations of [PSR-7](https://www.php-fig.org/psr/psr-7/); the primary purpose is
 to provide constants for referring to request methods, response status codes and
 messages, and potentially common headers.