FYI I have made the PR here: Add LinearAlgebra to make example with `norm` work by nilshg · Pull Request #2504 · JuliaPlots/Plots.jl · GitHub
@TheLateKronos in this case as Tamas said it was enough to use the GitHub web interface - click Fork on the Plots repo (top right corner), then navigate to the file in question in your forked repo (here shorthands.jl), click the little edit pen symbol on the top of your file, make the change you want, save it, then press Pull request on the overview page of your forked repo and you’re done.