fix(readme): update modalPresentationStyle to 'fullscreen'

[tcodes0]

fix(readme): update modalPresentationStyle to 'fullscreen' to fix use case of pasting code from readme and seeing native crash

PR Checklist

• The PR title follows our guidelines: https://github.com/proyecto26/react-native-inappbrowser/blob/master/CONTRIBUTING.md#pull-request-process.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• All existing tests are passing
• Tests for the changes are included

What is the current behavior?

For my use case, pasting readme code into my project crashes immediately on attempting to open the browser

found this: #130 (comment)

I'm not familiar with why only fullscreen is supported in my use case. It may or may not make sense to update readme. Sending PR as it is really small change with a big potential impact. Apparently prettier changed some whitespace too.

System:
    OS: macOS Mojave 10.14.6
    CPU: (8) x64 Intel(R) Core(TM) i7-6700K CPU @ 4.00GHz
    Memory: 93.93 MB / 16.00 GB
    Shell: 5.0.17 - /usr/local/bin/bash
  Binaries:
    Node: 14.4.0 - ~/.nvm/versions/node/v14.4.0/bin/node
    Yarn: 1.22.4 - /usr/local/bin/yarn
    npm: 6.14.5 - ~/.nvm/versions/node/v14.4.0/bin/npm
    Watchman: 4.9.0 - /usr/local/bin/watchman
  Managers:
    CocoaPods: 1.9.3 - /usr/local/bin/pod
  SDKs:
    iOS SDK:
      Platforms: iOS 13.2, DriverKit 19.0, macOS 10.15, tvOS 13.2, watchOS 6.1
    Android SDK:
      API Levels: 21, 22, 24, 25, 26, 27, 28
      Build Tools: 23.0.1, 23.0.2, 26.0.1, 26.0.2, 26.0.3, 27.0.2, 27.0.3, 28.0.0, 28.0.3, 29.0.0, 29.0.1
      System Images: android-21 | Google APIs Intel x86 Atom_64, android-28 | Google APIs Intel x86 Atom, android-28 | Google APIs Intel x86 Atom_64
      Android NDK: Not Found
  IDEs:
    Android Studio: 3.4 AI-183.6156.11.34.5522156
    Xcode: 11.3.1/11C504 - /usr/bin/xcodebuild
  Languages:
    Java: 1.8.0_202-ea - /usr/bin/javac
    Python: 2.7.17 - /usr/local/bin/python
  npmPackages:
    @react-native-community/cli: Not Found
    react: 16.13.1 => 16.13.1
    react-native: 0.62.2 => 0.62.2
  npmGlobalPackages:
    *react-native*: Not Found

What is the new behavior?

Fixes/Implements/Closes #[Issue Number].