Fix code snippet in getting started guide (#434)

Description of changes

Updates a code snippet from the getting started guide that demonstrated an improper way of disposing of webview panel resources when the panel is closed.

docs/getting-started.md

@@ -157,7 +157,7 @@ With the `dispose` method defined, you also need to update the constructor metho
 private constructor(panel: vscode.WebviewPanel) {
     // ... other code ...
 
-    this._panel.onDidDispose(this.dispose, null, this._disposables);
+    this._panel.onDidDispose(() => this.dispose(), null, this._disposables);
 }
 ```