Update hello_world_plugin.mdx (elastic#197104)

## Summary

Changed import path for Kibana Core Public imports, previous was not
working. New import path reflects what I found elsewhere in the
codebase, and works.

dev_docs/getting_started/hello_world_plugin.mdx

@@ -90,7 +90,7 @@ And add the following to it:
 ```ts
 import React from 'react';
 import ReactDOM from 'react-dom';
-import { AppMountParameters, CoreSetup, CoreStart, Plugin } from '../../../src/core/public';
+import { AppMountParameters, CoreSetup, CoreStart, Plugin } from '@kbn/core/public';
 
 export class HelloWorldPlugin implements Plugin {
   public setup(core: CoreSetup) {