fix doc

Author: CarloLucibello

docs/src/messagepassing.md

@@ -15,7 +15,7 @@ The message passing corresponds to the following operations
 \end{aligned}
 ```
 where ``\phi`` is expressed by the [`compute_message`](@ref) function, 
-``\gamma_x`` and ``\gamma_v`` by [`update_node`](@ref) and [`update_edge`](@ref)
+``\gamma_x`` and ``\gamma_e`` by [`update_node`](@ref) and [`update_edge`](@ref)
 respectively.
 
 See [`GraphConv`](ref) and [`GATConv`](ref)'s implementations as usage examples.