docs(firebase_messaging): update the readme.md file

[Abhishek01039]

Description

The portion of the Application.java file is not working. After some research I have got the working code which I have updated it.

Related Issues

#3952

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[Abhishek01039]

Hello @Ehesp, @Salakar and @helenaford
is there any issue in code because I have changed only the java part of readme.md file?
Thanks

[Abhishek01039]

Hello @Salakar
have you updated this portion along with your PR?
Thanks