Add missing screenOptions to Tab.Navigator

[pooriamo]

When I copied the code from "A native stack navigator for each tab", I realized the header is being duplicated. I checked that in the snack version, there is this "screenOptions" which is missing in the docs. I added it now.

READ ME PLEASE!

TL;DR: Make sure to add your changes to versioned docs

Thanks for opening a PR!

The docs cover several versions of react-navigation, and in some cases there are several files (for version 1, version 2 and etc.) that all describe a single page of the docs (eg. "Getting Started").

Please make sure that the edit you're making in docs/file-you-edited.md is also included in the file for the correct version, eg. /versioned_docs/version-3.x/file-you-edited.md for version 3. If such file doesn't exist, please create it. :+1:

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