Corrects shrink package name in `Fuzz.custom` docs
Corrects shrink package name in `Fuzz.custom` docs

The current docs for `Fuzz.custom` point to the wrong shrink package -- looks like `elm-test` changed from `elm-community/shrink` to `eeue56/elm-shrink` a few months back but but this bit of documentation wasn't corrected. This PR just corrects the docs to point to the right package.
elm-community/elm-test