Feynman rule convention #116
Description
in the Feynman rule docs, https://numericaleft.github.io/FeynmanDiagram.jl/dev/manual/feynman_rule/, we are currently using G^{-1}=g^{-1}+\Sigma. However, in many books, it should be G^{-1}=g^{-1}-\Sigma in books.