Installation Instructions - Prompt Character

This isn't super high priority as it's just a user interface issue, but it would be a quick fix.

When first installing the keybase command-line tool, I was sent to the prerequisites page. I typed "node" to make sure that I already had Node.js installed. Node returned with:

```
$ node
>
```

Great! I quit node and then moved on to the installation instructions page, which started with:

```
# some of these may require `sudo` in front,
# if you installed Node as root
# -----------------------------------------

> npm install -g keybase-installer # get the installer
> keybase-installer # run the installer, which verifies...
> keybase version # it worked!
```

Huh! I guess you type these into the Node prompt? Obviously that failed, and the ensuing attempt to run them on the command-line was successful. I know my initial confusion reveals me to be a Node.js noob (and a keyword scanner instead of an attentive reader), but it would really help disambiguate things if the prompt character was a $ instead.

(At least for Linux and Mac OS X. Maybe the $ would be confusing for Windows users? A platform-agnostic alternative might be to establish context with one sentence that says the following should be run on command line, followed by a second sentence containing the content of the top three comment lines, which could then removed from the code block.)

Open the prerequisites and installation instructions page described in the issue, then review the command examples and their prompt characters. Make the shell context unambiguous for new installers, and confirm that the displayed commands are clearly meant to run outside the Node.js prompt.

Written by the indexing model from the issue text.