Update lineplot syntax in activations.jl docs to work with new UnicodePlots.jl

[zengmao]

UnicodePlots.jl version 3.5 (April 2023) changed the lineplot syntax. See the changelog:
https://github.com/JuliaPlots/UnicodePlots.jl/blob/main/CHANGELOG.md

Now the syntax is lineplot(-1, 1, x->x^2) instead of lineplot(x->x^2, -1, 1). The function plots in activations.jl doc strings still uses the old syntax.

[mcabbott]

Oh no, that's weird? Functions first is near-universal, to allow do blocks. Seems to be here, no discussion & not marked as breaking: JuliaPlots/UnicodePlots.jl#343

Anyway we couldn't get these to work as doctests, so at least nothing broke here! We can change them all if someone wants to. Thanks for the note.