Update README examples
Update README examples

Looks like RDatasets had some trivial changes to column names, fix
examples in README for clarity

zhmz90/Gadfly.jl,Godisemo/Gadfly.jl,peter1000/Gadfly.jl,joehuchette/Gadfly.jl,spencerlyon2/Gadfly.jl,jiahao/Gadfly.jl,maximsch2/Gadfly.jl,mbauman/Gadfly.jl,tbreloff/Gadfly.jl,Roger-luo/Gadfly.jl,kshramt/Gadfly.jl,JuliaPackageMirrors/Gadfly.jl,dcjones/Gadfly.jl