update add_self_loops docstring

Author: CarloLucibello

src/GNNGraphs/transform.jl

@@ -41,17 +41,17 @@ end
 """
     add_self_loops(g::GNNHeteroGraph, edge_t::EType)
 
-If the source node type is the same as destination node type in `edge_t`,
-return a graph with the same features as `g` but also adding self-loops 
-of the specified type, `edge_t`. Otherwise it returns `g` unchanged.
+If the source node type is the same as the destination node type in `edge_t`,
+return a graph with the same features as `g` but also add self-loops 
+of the specified type, `edge_t`. Otherwise, it returns `g` unchanged.
 
-Nodes with already existing self-loops of type edge_t will obtain 
-a second self-loop of type edge_t.
+Nodes with already existing self-loops of type `edge_t` will obtain 
+a second set of self-loops of the same type.
 
-If the graphs has edge weights for edges of type edge_t, the new edges will have weight 1.
+If the graph has edge weights for edges of type `edge_t`, the new edges will have weight 1.
 
-If no edges of type edge_t exist, or all existing edges have no weight, 
-then all new self loops will have no weight.
+If no edges of type `edge_t` exist, or all existing edges have no weight, 
+then all new self-loops will have no weight.
 """
 function add_self_loops(g::GNNHeteroGraph{Tuple{T, T, V}}, edge_t::EType) where {T <: AbstractVector{<:Integer}, V}
     function get_edge_weight_nullable(g::GNNHeteroGraph{<:COO_T}, edge_t::EType)