Use semantic button for event handler example (#2781)

As a general rule, `onClick` handlers should not be applied to non-interactive elements like a `div`. There is not substantive change to the code sample, but it doesn't provide a better example of semantic HTML for developers reading the documentation.

content/docs/error-boundaries.md

@@ -152,7 +152,7 @@ class MyComponent extends React.Component {
     if (this.state.error) {
       return <h1>Caught an error.</h1>
     }
-    return <div onClick={this.handleClick}>Click Me</div>
+    return <button onClick={this.handleClick}>Click Me</button>
   }
 }
 ```