Update ref-03-component-specs.md

Edit the description of componentDidMount() to make clear that the function is called only once during the lifecycle of the component, after only the initial render() call. The description is now similar to that for componentWillMount(). The existing wording does not make it clear that the function is called only after the initial render. (Granted, it is possible to infer this from descriptions of the component lifecycle found elsewhere in the documentation, but I think explicit is better than implicit in this case).