fix syntax error

[liangfeidotme]

Changed Currently the Sky Engine assumes the entry point for your application is a main function is located inside a main.sky file at the root of the package to Currently the Sky Engine assumes the entry point for your application is a main function which is located inside a main.sky file at the root of the package.

[eseidelGoogle]

Thank you very much for the pull request. Unfortunately this repository is automatically generated from the mojo repository. Furthermore the mojo repository is not yet able to accept pull requests (I'm working on it), but rather has as separate contribution process:
https://github.com/domokit/mojo/tree/master#contribute