sym vs symm

Stream: graph theory

[Kyle Miller]

Here's a small task someone with time on their hands could do (if others like @Bhavik Mehta, @Alena Gusakov, or @Eric Wieser are on board with this idea). It seems like it would be more consistent with mathlib if simple_graph.sym were simple_graph.symm.

[Kyle Miller]

#9134

[Yaël Dillies]

Good change