Fix ref in the docs (#410)

* Change ref to link to Flux DataLoader

* Update docs/src/heterograph.md

---------

Co-authored-by: Carlo Lucibello <[email protected]>

Author: aurorarossi

docs/src/heterograph.md

@@ -115,7 +115,7 @@ GNNHeteroGraph:
   num_edges: Dict((:A, :to, :B) => 640, (:B, :to, :A) => 640)
   num_graphs: 32
 ```
-Batching is automatically performed by the [`DataLoader`](@ref) iterator
+Batching is automatically performed by the [`DataLoader`](https://fluxml.ai/Flux.jl/stable/data/mlutils/#MLUtils.DataLoader) iterator
 when the `collate` option is set to `true`.
 
 ```jldoctest