docs updated for firebase_messaging

[srsandy]

firebase_messaging

Description

Updating package/firebase_messaging Readme.md file. I'm not an Android or an IOS developer. Just started with flutter. During the time I was adding firebase_messaging to the project. I came across "Optionally handle background messages". I was not able to understand clearly what to do in this section. I've done a small update in the docs so if someone, not a mobile developer lands on the firebase_messaging. They still be able to understand. I will add the related issue regarding this PR. Thank You.

Related Issues

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[kroikie]

Thanks! @srsandy

[misaelriojasftf]

not sure I think was understandable. but good noticed

[kroikie]

@srsandy sorry for taking so long to get to this PR. We are in the process of updating the README for this plugin so your change will be incorporated there. Thanks for the PR!