Update README: useBuiltins: true is changed to "entry"

[exarus]

babel-preset-env@next founds "useBuiltins": true is illegal. README is updated accordingly.

Q	A
Fixed Issues?	❌
Patch: Bug Fix?	❌
Major: Breaking Change?	❌
Minor: New Feature?	❌
Tests Added + Pass?	❌
Documentation PR	👍
Any Dependency Changes?	❌

I've checked @babel/preset-env@next and found that "useBuiltins": true is illegal. Considering that:

1. "useBuiltins": true doesn't work in @next version
2. "useBuiltins": true is not documented
3. "useBuiltins": "entry" works in @latest version

I suggest changing "useBuiltins": true to "useBuiltins": "entry"

Also I suggest that README is future-oriented because it specifies @babel/preset-env npm package that is currently absent in npm.

Special thanks to @existentialism for explaining the "useBuiltins": true option.

[existentialism]

@exarus the README is future-oriented, we're moving to scoped packages, so @babel/preset-env is correct, it just hasn't been published yet.