fix reference

Author: CarloLucibello

docs/src/datasets/graphs.md

@@ -3,7 +3,7 @@
 A collection of datasets with an underlying graph structure.
 Some of these datasets contain a single graph, that can be accessed
 with `dataset[]` or `dataset[1]`. Others contain many graphs, 
-accessed through `dataset[i]`. Graphs are represented by the [`Graph`](@ref) type.
+accessed through `dataset[i]`. Graphs are represented by the [`MLDatasets.Graph`](@ref) type.
 
 ## Index