Update lecture.md

[Kunz-David]

Adding environment for the following to work:

a = LessVaguePosition(1,2)
a.x = 2

Now it gives:

julia> a = LessVaguePosition(1,2)
ERROR: UndefVarError: LessVaguePosition not defined

[Kunz-David]

I hope I understand this, but I think the a = LessVaguePosition(1,2) is not evaluating correctly.

[nmheim]

@pevnak do you want to take a look at this? :)

[pevnak]

I will check it next week