Fix clippy lint needless_doctest_main

[jayvdb]

Fixes these clippy lints

warning: needless `fn main` in doctest
  --> src/clients/blocking_client.rs:63:9
   |
63 |       /// use mini_redis::clients::BlockingClient;
   |  _________^
64 | |     ///
65 | |     /// fn main() {
66 | |     ///     let client = match BlockingClient::connect("localhost:6379") {
...  |
70 | |     /// # drop(client);
71 | |     /// }
   | |_________^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_doctest_main
   = note: `#[warn(clippy::needless_doctest_main)]` on by default