Link to mathlib(3) in "How to contribute to mathlib"

Stream: mathlib4

[Michael Stoll]

I just noticed that the text "You can view the output by visiting https://github.com/leanprover-community/mathlib/tree/my_new_branch" on the How to contribute to mathlib page seems to (still) link to mathlib3. It would make sense to change this.

[Patrick Massot]

Fixed, thanks @Michael Stoll!