Update messaging.md

[hiepxanh]

Checklist

• Issue number for this PR: #nnn (required)
• Docs included?: (yes)
• Test units included?: (no)
• In a clean directory, yarn install, yarn test run successfully? yes

Description

Updating the documentation:
Because of this issue comment, I see that we unable to receive messaging if we have different firebase version in firebase-messaging-sw.js

refer to:
#2299 (comment) #2299 (comment)
#1904 (comment)

And this issue #1904 (comment) have quote:

I did tried others many versions of both and theirs convinations and nothing seem to worked, so i registerd a firebase-messaging-sw.js

solution was confirmed in stackoverflow:
https://stackoverflow.com/questions/40411059/create-firebase-notification-with-page-in-foreground-focus
https://stackoverflow.com/questions/58279900/angular-7-cannot-receive-firebase-notification-in-the-foreground
https://stackoverflow.com/questions/58386934/web-firebase-messaging-onmessage-not-fired-but-background-notification-perfec

So, we need have an example and warning developer to keep it up to date with firebase in package.json

Code sample

quick view to preview md is here: PR preview docs