docs: simplify helloworld-tutorial (#1369)

- Remove the `hello_world` crate prefix, `HelloReply` is imported directly.
- Remove useless calls to `into()`:

error: useless conversion to the same type: `std::string::String`
  --> src/main.rs:18:22
   |
18 |             message: format!("Hello {}!", request.into_inner().name).into(),
   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into()`: `format!("Hello {}!", request.into_inner().name)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_conversion
   = note: `-D clippy::useless-conversion` implied by `-D warnings`

Co-authored-by: tottoto <tottotodev@gmail.com>

examples/helloworld-tutorial.md

@@ -166,7 +166,7 @@ impl Greeter for MyGreeter {
     ) -> Result<Response<HelloReply>, Status> { // Return an instance of type HelloReply
         println!("Got a request: {:?}", request);
 
-        let reply = hello_world::HelloReply {
+        let reply = HelloReply {
             message: format!("Hello {}!", request.into_inner().name), // We must use .into_inner() as the fields of gRPC requests and responses are private
         };
 
@@ -215,7 +215,7 @@ impl Greeter for MyGreeter {
     ) -> Result<Response<HelloReply>, Status> {
         println!("Got a request: {:?}", request);
 
-        let reply = hello_world::HelloReply {
+        let reply = HelloReply {
             message: format!("Hello {}!", request.into_inner().name),
         };