Updates site description in MkDocs configuration

Author: froschdesign

mkdocs.yml

@@ -29,6 +29,6 @@ pages:
     - "_reference-serialization": serialization.md
     - "_reference-api": api.md
 site_name: zend-diactoros
-site_description: 'zend-diactoros: PSR-7 HTTP message implementation'
+site_description: 'PSR-7 HTTP message implementations.'
 repo_url: 'https://github.com/zendframework/zend-diactoros'
 copyright: 'Copyright (c) 2016-2018 <a href="https://www.zend.com/">Zend Technologies USA Inc.</a>'