demo: Return a more identifying string for GET requests

It's good practice to return something identifying the service
offered on basic requests. This could include a link to the
paper and/or repository after publication.

demo/examples/server.rs

@@ -92,7 +92,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 }
 
 async fn handler() -> &'static str {
-    "Hello, Client!"
+    "Boomerang protocol server. Try POST /\n"
 }
 
 lazy_static! {