Update link to homepage in composer.json

This ensure that visitors from Packagist do not get an SSL certificate
warning as GitHub Pages does not create an SSL certificate with both the
apex domain and www subdomain registered.

composer.json

@@ -3,7 +3,7 @@
     "type": "library",
     "description": "Slim is a PHP micro framework that helps you quickly write simple yet powerful web applications and APIs",
     "keywords": ["framework","micro","api","router"],
-    "homepage": "https://slimframework.com",
+    "homepage": "https://www.slimframework.com",
     "license": "MIT",
     "authors": [
         {