Update README.md - container.has() is not a function

[abythell]

In version 6.0.0, container.has() was renamed container.hasRegistration(), but the README.md was not updated accordingly.

I didn't think this minor change warranted a pull request, but it would be a helpful update for those migrating from v5 who don't first read the changelog (not that I speak from experience or anything...)

[jeffijoe]

Damn, I thought I had found every reference.