Fix a few miscellaneous typos (Unkown -> Unknown, Recieve -> Receive)

Docker-DCO-1.0-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
google-compute-engine: fix typos
Fix for #3345: Add Content-Type Header "application/json" to GET /version and /info responses in remote API

Docker-DCO-1.0-Signed-off-by: Manuel Woelker <github@manuel.woelker.org> (github: manuel-woelker)
Hello World: fix detach behavior

The hello world example relies on being able to exit the container attachment using Control-C without stopping it. According to this thread (and my own experience), this doesn't work anymore:

https://groups.google.com/forum/#!msg/docker-user/nWXAnyLP9-M/kbv-FZpF4rUJ

Added -sig-proxy=false to the attach command.
Add the customary bold
Merge pull request #3402 from manuel-woelker/3345-add-content-type-header-to-version-remote-api

Fix for #3345: Add Content-Type Header "application/json" to GET /version response in remote API
Merge pull request #3499 from tianon/typos

Fix a few miscellaneous typos (Unkown -> Unknown, Recieve -> Receive)
Merge pull request #3501 from proppy/patch-1

google-compute-engine: fix typos
Merge pull request #3503 from gertvv/patch-1

Hello World: fix detach behavior