Docs: remove dollar sign to make instructions easier to copy and paste

[sarafonseca-123]

Remove dollar sign to make instructions easier to copy and paste. Other sections on the document already had the $ removed, like From Apt.

[scottrigby]

I'm a little hesitant to make the v2 docs easier to copy/paste, especially given some things won't work anymore. For example, this would need to be updated to work:

cd $GOPATH
mkdir -p src/k8s.io
cd src/k8s.io
git clone https://github.com/helm/helm.git
cd helm
make bootstrap build

and I know there's a lot more in the helm 2 instructions that no longer work. Do we really want to go down this route to make the v2 docs fully usable?

[scottrigby]

also I meant to say thanks @sarafonseca-123 for the PR and appreciate contributions! There are some other areas of the docs we could really use some community help with if you'd like to dig in 🙂