readme(camera) make `modalPresentationStyle` option visible

[tralves]

Small fix. Luckily I found this option in the code and it was just what I needed!

[cla-bot[bot]]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file. In order for us to review and merge your code, please sign the CLA at https://www.nativescript.org/cla. CLA has not been signed by users: @tralves. After signing the CLA, you can ask me to recheck this PR by posting @cla-bot check as a comment to the PR.

[tralves]

@cla-bot check

[cla-bot[bot]]

The cla-bot has been summoned, and re-checked this pull request!

[NathanWalker]

Thanks @tralves 🤗