Fix broken link. (#861)

content/warnings/special-props.md

@@ -6,4 +6,4 @@ permalink: warnings/special-props.html
 
 Most props on a JSX element are passed on to the component, however, there are two special props (`ref` and `key`) which are used by React, and are thus not forwarded to the component.
 
-For instance, attempting to access `this.props.key` from a component (i.e., the render function or [propTypes](https://reactjs.org/docs/typechecking-with-proptypes.html#proptypes)) is not defined. If you need to access the same value within the child component, you should pass it as a different prop (ex: `<ListItemWrapper key={result.id} id={result.id} />`). While this may seem redundant, it's important to separate app logic from reconciling hints.
+For instance, attempting to access `this.props.key` from a component (i.e., the render function or [propTypes](/docs/typechecking-with-proptypes.html#proptypes)) is not defined. If you need to access the same value within the child component, you should pass it as a different prop (ex: `<ListItemWrapper key={result.id} id={result.id} />`). While this may seem redundant, it's important to separate app logic from reconciling hints.