[rust] Correct panic message in executor.

Change-Id: Ie3cd6330d083ac471d66966b24dabd4a27946a5f

src/lib/fuchsia-async/src/executor.rs

diff --git a/src/lib/fuchsia-async/src/executor.rs b/src/lib/fuchsia-async/src/executor.rs
index 80fe39dc6..18c3584 100644
--- a/src/lib/fuchsia-async/src/executor.rs
+++ b/src/lib/fuchsia-async/src/executor.rs
@@ -614,7 +614,7 @@
              Use `run_singlethreaded` instead.",
         );
         if let Some(_) = self.next_packet {
-            panic!("Error: called `run_singlethreaded` on an executor with a packet waiting");
+            panic!("Error: called `run` on an executor with a packet waiting");
         }
 
         let pair = Arc::new((Mutex::new(None), Condvar::new()));