Fix helloworld tutorial.

updated tutorial to use tokio 1.x.

Author: 16yuki0702

examples/helloworld-tutorial.md

@@ -115,7 +115,7 @@ path = "src/client.rs"
 [dependencies]
 tonic = "0.4"
 prost = "0.7"
-tokio = { version = "0.2", features = ["macros"] }
+tokio = { version = "1.0", features = ["macros"] }
 
 [build-dependencies]
 tonic-build = "0.4"
@@ -179,7 +179,7 @@ impl Greeter for MyGreeter {
 Finally, let's define the Tokio runtime that our server will actually run on. This requires Tokio to be added as a dependency, so make sure you included that!
 
 ```rust
-#[tokio::main]
+#[tokio::main(flavor = "current_thread")]
 async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let addr = "[::1]:50051".parse()?;
     let greeter = MyGreeter::default();
@@ -224,7 +224,7 @@ impl Greeter for MyGreeter {
     }
 }
 
-#[tokio::main]
+#[tokio::main(flavor = "current_thread")]
 async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let addr = "[::1]:50051".parse()?;
     let greeter = MyGreeter::default();
@@ -272,7 +272,7 @@ pub mod hello_world {
 The client is much simpler than the server as we don't need to implement any service methods, just make requests. Here is a Tokio runtime which will make our request and print the response to your terminal:
 
 ```rust
-#[tokio::main]
+#[tokio::main(flavor = "current_thread")]
 async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let mut client = GreeterClient::connect("http://[::1]:50051").await?;
 
@@ -298,7 +298,7 @@ pub mod hello_world {
     tonic::include_proto!("helloworld");
 }
 
-#[tokio::main]
+#[tokio::main(flavor = "current_thread")]
 async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let mut client = GreeterClient::connect("http://[::1]:50051").await?;