ports.7: Update examples with install-missing-packages

[0mp]

Summary

ports.7: Update examples with install-missing-packages

The ports framework recently grew support for installing dependencies with a
dedicated target called "install-missing-packages". Let's retire the carefully
constructed one-liner that were used for this purpose before and use the
official target instead.

[bcr]

Cool new feature. Time to document it. Approved!

[0mp]

Thanks, @bcr!