Update typechecking-with-proptypes.md (#3513)

Typo: "allow PropTypes to be proper applied" => "allow PropTypes to be properly applied"

Author: DaniBedz

content/docs/typechecking-with-proptypes.md

@@ -199,7 +199,7 @@ The `defaultProps` will be used to ensure that `this.props.name` will have a val
 
 ### Function Components {#function-components}
 
-If you are using function components in your regular development, you may want to make some small changes to allow PropTypes to be proper applied.
+If you are using function components in your regular development, you may want to make some small changes to allow PropTypes to be properly applied.
 
 Let's say you have a component like this: