fixed typo

Author: fabpot

src/Symfony/Component/Serializer/README.md

@@ -1,7 +1,7 @@
 Serializer Component
 ====================
 
-With the Serializer component its possible to handle serializing data structures,
+With the Serializer component it's possible to handle serializing data structures,
 including object graphs, into array structures or other formats like XML and JSON.
 It can also handle deserializing XML and JSON back to object graphs.