Update link and shorter example

[thaJeztah]

the link to the userguide was updated, so updating
it in the hello-world image to save a redirect.

also removed "bash" from the example, because it's
the default for the ubuntu image.

[thaJeztah]

ping @tianon I wasn't sure if you wanted to keep the bash in the ubuntu example (or even change it to docker run -it alpine sh), so happy to hear :-)

[tianon]

Ah nice! I do like keeping bash in here since it helps reinforce the concept of docker run <image> <command> really early on without having to have users think about default commands. 👍

(and I'm also a fan of leaving this on ubuntu specifically because it's a name most users have heard before, so it's a good "second intro image" -- otherwise I would've switched it to debian ages ago 🙈)

[thaJeztah]

Yes, I also thought about that; having them run ubuntu once, warms up their image cache for a better experience in further examples. Let me change that line back

[thaJeztah]

updated 👍

[tianon]

LGTM 👍

Thanks! 😄