docs: mark code snippets with their language for more correct syntax highlighting

[zarvox]

https://docs.sandstorm.io/en/latest/vagrant-spk/installation/#install-vagrant-spk has some code snippets that are getting syntax-highlighted with probably the wrong language. The shell snippet at the end appears to be using C++-style // comments, and the various shell output bits are getting random-looking formatting applied.

In standard Github-flavored-markdown, you can write

```bash
mkdir -p ~/projects

to hint the desired language for code highlighting; let's use that for code blocks.  For the shell output, perhaps we just want

VirtualBox: Command not found.

or maybe there's a "none" syntax highlighter we could try?

[paulproteus]

text

probably is good enough for uncolored output. I could try it.