style(combinatorics/simple_graph/basic): edit proof of lemma to match style guidelines (#5245)
style(combinatorics/simple_graph/basic): edit proof of lemma to match style guidelines (#5245)

Rewrite proof of `adj_iff_exists_edge` to match style guidelines.



Co-authored-by: agusakov <067de2d7f4f5809454de1022e9c3aab9efe72888@users.noreply.github.com>

leanprover-community/mathlib,leanprover-community/mathlib