Update Linear.jl Example

[jrock2468]

Fixes error message "ERROR: TypeError: in typeassert, expected MultiJuMP.MultiMethod, got Symbol".

[matbesancon]

thanks @jrock2468 Could you add a compat field for JuMP at the same time in the Project.toml:

[compat]
JuMP = "0.18"

[amgad-naiem]

Already updated with another PR