changed the link to clone the library (#4725)

Doing `git clone -b v2 git@github.com:wix/react-native-navigation.git` gives me this:
```
fatal: Could not read from remote repository.
Please make sure you have the correct access rights and the repository exists.
```

docs/docs/showcases.md

 The playground example is a place where we showcase and test all features of react-native-navigation.
 
 ```bash
-git clone -b v2 git@github.com:wix/react-native-navigation.git
+git clone https://github.com/wix/react-native-navigation
 cd react-native-navigation
 npm install
 npm run xcode # Opens XCode