[guide] variable binding outside a function

[evangoer]

If the reader starts section 4 and tries to play around with variables by doing a bare let x = 5i;, they'll run into the error:

variables.rs:1:1: 1:4 error: expected item, found `let`

The guide should update the first few examples in section 4 to explicitly show let declared inside a function, so that the code will compile. This would also be a good place to explain why declaring the let outside fn yields an error.

[evangoer]

Well done, thank you all! :)