Change prompt symbol in comments from $ to %. Close #334.

[Kramed](https://github.com/GitbookIO/kramed), gitbook’s Markdown parser, see the dollar signs in the comments of [fs.rs](https://github.com/rust-lang/rust-by-example/blob/master/examples/fs/fs.rs#L5) as `MathJax` delimiters:

```rust
// A simple implementation of `$ cat path`

// A simple implementation of `$ echo s > path`

// A simple implementation of `$ touch path` (ignores existing files)
```

Disabling MathJax, as it is not used, in [book.json](https://github.com/rust-lang/rust-by-example/blob/master/book.json) as mentionned [here](https://www.npmjs.com/package/gitbook-plugin-mathjax)...

```json
{
    "plugins": ["-mathjax", "rust-playpen"],
    "pluginsConfig": {
        "rust-playpen": {}
    }
}
```

...don’t work. The library is not loaded anymore, but the setting is not passed to `kramed`. Disabling MathJax by hand in [kramed.js](https://github.com/GitbookIO/kramed/blob/master/lib/kramed.js#L1333) does, but is obviously impractical, and dirty.

MathJax does allow to [change delimiters](http://docs.mathjax.org/en/latest/options/tex2jax.html#configure-tex2jax) but the `$$` pattern is hard-coded in a regex inside [kramed.js](https://github.com/GitbookIO/kramed/blob/master/lib/kramed.js#L498).

I suggest we just change the `$`s to `#`s. That way we’ll keep the semantic that code in comments are inside backticks, which is good, and used in other examples.