add no run to new example

Author: hacknus

library/std/src/path.rs

@@ -2550,7 +2550,7 @@ impl Path {
     /// ```
     ///
     /// if the path is a directory, the function will always return the complete filename
-    /// ```
+    /// ```no_run
     /// use std::path::Path;
     ///
     /// assert_eq!("2024.11.23_directory", Path::new("2024.11.23_directory").file_stem().unwrap());