docs(README.md): replace deprecated findDOMNode with callback ref example

Author: EndiM

README.md

@@ -97,11 +97,10 @@ Check the example [React-tooltip Test](https://react-tooltip.netlify.com/)
 > Hide the tooltip manually, the target is optional, if no target passed in, all existing tooltips will be hidden
 
 ```js
-import {findDOMNode} from 'react-dom'
 import ReactTooltip from 'react-tooltip'
 
-<p ref='foo' data-tip='tooltip'></p>
-<button onClick={() => { ReactTooltip.hide(findDOMNode(this.refs.foo)) }}></button>
+<p ref={ref => this.fooRef = ref} data-tip='tooltip'></p>
+<button onClick={() => { ReactTooltip.hide(this.fooRef) }}></button>
 <ReactTooltip />
 ```
 
@@ -114,11 +113,10 @@ import ReactTooltip from 'react-tooltip'
 > Show specific tooltip manually, for example:
 
 ```js
-import {findDOMNode} from 'react-dom'
 import ReactTooltip from 'react-tooltip'
 
-<p ref='foo' data-tip='tooltip'></p>
-<button onClick={() => { ReactTooltip.show(findDOMNode(this.refs.foo)) }}></button>
+<p ref={ref => this.fooRef = ref} data-tip='tooltip'></p>
+<button onClick={() => { ReactTooltip.show(this.fooRef) }}></button>
 <ReactTooltip />
 ```