Fix single vs plural (#8738)

Maybe it's not very important, just misprint fix

docs/typechecking-with-proptypes.md

@@ -107,7 +107,7 @@ MyComponent.propTypes = {
 };
 ```
 
-### Requiring Single Children
+### Requiring Single Child
 
 With `React.PropTypes.element` you can specify that only a single child can be passed to a component as children.