fixed typo

Author: fabpot

README.md

@@ -5,7 +5,7 @@ What is Symfony2?
 -----------------
 
 Symfony2 is a PHP 5.3 full-stack web framework. It is written with speed and
-flexibility in mind. It allows developers to built better and easy to maintain
+flexibility in mind. It allows developers to build better and easy to maintain
 websites with PHP.
 
 Symfony can be used to develop all kind of websites, from your personal blog