rm unneeded closure reference in TransformMap docs
//! let src = Array3::fill(extent, 0);
//! let chunk_shape = Point3i::fill(4);
//! let mut dst = ChunkMap3x1::build_with_hash_map_storage(chunk_shape);
-//! let tfm = TransformMap::new(&src, &|value: i32| value + 1);
+//! let tfm = TransformMap::new(&src, |value: i32| value + 1);
//! copy_extent(&extent, &tfm, &mut dst);
//! ```
