Fix documentation for AssetReader::is_directory function (#11538)

# Objective

- Fix documentation for `AssetReader::is_directory` (it is currently
exactly the same as docs for `read_directory`)

---------

Co-authored-by: Kanabenki <lucien.menassol@gmail.com>

crates/bevy_asset/src/io/mod.rs

@@ -78,7 +78,7 @@ pub trait AssetReader: Send + Sync + 'static {
         &'a self,
         path: &'a Path,
     ) -> BoxedFuture<'a, Result<Box<PathStream>, AssetReaderError>>;
-    /// Returns an iterator of directory entry names at the provided path.
+    /// Returns true if the provided path points to a directory.
     fn is_directory<'a>(
         &'a self,
         path: &'a Path,