Fixed typo in options description (#4597)

Resources/doc/reference/form_types.rst

@@ -252,7 +252,7 @@ to_string_callback
     ;
 
 multiple
-  defaults to false. Set to true, if you're field is in many-to-many relation.
+  defaults to false. Set to true, if your field is in a many-to-many relation.
 
 placeholder
   defaults to "". Placeholder is shown when no item is selected.