Update README.md

Fixed a comment in the sample code.

README.md

@@ -20,7 +20,7 @@ But most of us are too lazy to check explicitly for every single return of every
 
 ```php
 // This code is incorrect. Twice.
-// "file_get_contents" can return false if the file does not exists
+// "file_get_contents" can return false if the file does not exist
 // "json_decode" can return false if the file content is not valid JSON
 $content = file_get_contents('foobar.json');
 $foobar = json_decode($content);