walkthrough: Outdated property name in JS Type Checking section

[xyzshantaram]

Does this issue occur when all extensions are disabled?: Yes

• VS Code Version: 1.58.0
• OS Version: Arch Linux x86_64 5.12.15-arch1-1

Steps to Reproduce:

1. Open the VSCode interactive playground.
2. Scroll down to the JavaScript Type Checking section.

Description:
The walkthrough section currently advises setting "javascript.implicitProjectConfig.checkJs": true to get TS checks in JavaScript files.

The property's been deprecated in favour of "js/ts.implicitProjectConfig.checkJs": true.

The walkthrough should be updated to reflect the same.