Update example code in documentation

[Shehriyar Qureshi]

The code in the first example

#[cfg(not(test))]
let url = "https://api.twitter.com";

gives a compiler error expected item after attributes (also raised in issue #122 ).

I fixed it by replacing the let statement with a const one. If this is the right way to do it, the documentation should be updated with the correct code.

[Florin Lipan]

@Shehriyarqureshi thanks for reporting this. I updated the documentation by adding an explicit fn main { ... } around the code: 9f74586

do note that Rust wraps doc tests inside fn main { ... } by default so that's kind of implied when reading Rust docs (except when there is an explicit fn main {} call). that's also why my doc tests would pass with this code.

in any case, you are still free to move these declarations outside of fn main { ... } or simply pass mockito::server_url() to your client during tests (if your client exposes a method to set the base URL).