Fix typo astaxie#876

Author: dastier

en/04.2.md

@@ -1,6 +1,6 @@
 # 4.2 Verification of inputs
 
-One of the most important principles in web development is that you cannot trust anything from client side user forms. You have to validate all incoming data before use it. Many websites are affected by this problem, which is simple yet crucial.
+One of the most important principles in web development is that you cannot trust anything from client side user forms. You have to validate all incoming data before using it. Many websites are affected by this problem, which is simple yet crucial.
 
 There are two ways of verifying form data that are in common use. The first is JavaScript validation on the front-end, and the second is server validation on the back-end. In this section, we are going to talk about server side validation in web development.