Correct `setState()` usage in tutorial

Summary:
If the new state depends on the previous state, if I remember correctly, it’s safer to use  `setState()` with a function argument to ensure we’re not reading from an outdated `state`.

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The tutorial suggests to use `setState()` with an object argument when the new state depends on the previous state. In such situations, it’s preferable to use a function to ensure the previous state is up-to-date.

Updates documentation only, so there are no additional tests. Rendering the site.

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Closes https://github.com/facebook/react-native/pull/13358

Differential Revision: D4852404

Pulled By: hramos

fbshipit-source-id: 834759e16bcfbd5a8de71bf0c56f2b154f3321e1

docs/State.md

@@ -25,7 +25,9 @@ class Blink extends Component {
 
     // Toggle the state every second
     setInterval(() => {
-      this.setState({ showText: !this.state.showText });
+      this.setState(previousState => {
+        return { showText: !previousState.showText };
+      });
     }, 1000);
   }