Snippets update

[gpokat]

Added explanatory comment about math package usage
Changed Main() entry point to Run() as per design and toolchain

This small update of front page code snippets will add
explanatory comment to highlight that the Math package is
an example and do not exist yet.
Also it delivers change of Main() to Run() to
highlight correct entry point for Carbon lang.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.