Update download path

Following the instructions on the Cargo homepage gave me:

```
rustup: WARNING: Downloading rustup.sh from http://www.rust-lang.org/rustup.sh is deprecated.
rustup: Use https://static.rust-lang.org/rustup.sh instead.
```

So I'm updating the docs [=
Merge pull request #25 from jaredonline/patch-1

Update download path