Fix the compilation error (#532)

Try `cargo run --verbose` after copying the code into a separate project. This example snippet does not compile and requires `serde` create to be present in the main.rs. So include the serde crate will fix the compilation error.

src/web/clients/api/rest-get.md

@@ -7,6 +7,7 @@ with [`reqwest::get`] to get list of all users who have marked a GitHub project
 
 ```rust,no_run
 #[macro_use]
+extern crate serde;
 extern crate serde_derive;
 extern crate reqwest;
 use reqwest::Error;