Remove v2 syntax in migration guide (#27)

Author: LeeLenaleee

docs/guide/migration.md

@@ -11,7 +11,7 @@ Make sure to also read the [Chart.js v3 migration guide](https://www.chartjs.org
 As described in the [getting started](getting-started.md#integration), it's now required to manually register this plugin, either globally:
 
 ```js
-Chart.plugins.register(ChartDeferred);
+Chart.register(ChartDeferred);
 ```
 
 or locally:
@@ -50,4 +50,4 @@ Chart.defaults.set('plugins.deferred', {
 
 #### Chart.js type declaration <Badge text="TS only"/>
 
-Chart.js v3 now provides TypeScript type declaration files bundled in the npm package so it's **not** anymore required to install the `@types/chart.js` package.
+Chart.js v3 now provides TypeScript type declaration files bundled in the npm package so it's **not** anymore required to install the `@types/chart.js` package.