Refer to tokio as "mainstream" instead of "popular"

exercises/24_async/README.md

@@ -5,7 +5,7 @@ It is particularly efficient for applications where many independent IO-operatio
 
 Rust provides the necessary primitives to do asynchronous programming in the language.
 However, Rust's standard library does not include a runtime.
-For these exercises, we will use the popular runtime called `tokio`.
+For these exercises, we will use the mainstream runtime called `tokio`.
 
 ## Further information
 

exercises/24_async/async1.rs

@@ -13,7 +13,7 @@ static CHORES_DONE: AtomicU8 = AtomicU8::new(0);
 
 fn main() {
     // Async tasks need to be executed by a "runtime", which is not provided by
-    // Rust's standard library. We use the popular "tokio" runtime here.
+    // Rust's standard library. Here, we use the mainstream runtime `tokio`.
     let rt = tokio::runtime::Builder::new_current_thread()
         .build()
         .unwrap();

solutions/24_async/async1.rs

@@ -13,7 +13,7 @@ static CHORES_DONE: AtomicU8 = AtomicU8::new(0);
 
 fn main() {
     // Async tasks need to be executed by a "runtime", which is not provided by
-    // Rust's standard library. We use the popular "tokio" runtime here.
+    // Rust's standard library. Here, we use the mainstream runtime `tokio`.
     let rt = tokio::runtime::Builder::new_current_thread()
         .build()
         .unwrap();