fix spelling

Author: Daksh14

tokio/src/sync/set_once.rs

@@ -357,7 +357,7 @@ impl<T> SetOnce<T> {
 
     /// Waits until set is called.
     ///
-    /// If the state failed to initalize it will return `None`.
+    /// If the state failed to initialize it will return `None`.
     pub async fn get_wait(&self) -> Option<&T> {
         let notify_fut = self.notify.notified();
         pin!(notify_fut);