Add dependencies via npm add in AMS guide

[DanSchlachter]

In https://github.com/SAP-samples/btp-developer-guide-cap/blob/main/documentation/xsuaa-to-ams/2-integrate-with-ams.md

Instead of manually adding lines to package.json (step 1 and 2) and then run npm install (step 6), it would be better to use npm add or npm add -D