fixes add_edges + adds tests

[svilupp]

Fixes https://github.com/CarloLucibello/GraphNeuralNetworks.jl/issues/334

Changes:

• Fixes a typo (= vs ==) in providing num_edges for new edges in HeteroGraph.
• Adds light tests for add_edges and fixed functionality
• Removes a small typo in the docstring

[svilupp]

This time I ran a correct test file (🤦 ) and I can confirm it passes!

[CarloLucibello]

Thanks for these two super quick PRs. I just tagged a new release