docs: fix prebuild snippet

[karimStekelenburg]

The code snippet to run the expo prebuild command assumes you have the following script entry in your package.json

...
"scripts": {
    "prebuild": "expo prebuild"
}
...

However, this isn't always the case. For example, bootstapping a project with npx create-expo-app won't add that entry to the package.json.

This PR makes the snippets more setup independent.

Signed-off-by: Karim Stekelenburg karim@animo.id