Remove link to Medley

[puredanger]

After the newly updated linked to update-keys and update-vals there is a link to the Medley GitHub repo which is no longer relevant:

'Change'	assoc assoc-in dissoc merge merge-with select-keys update update-in (clojure.set/) rename-keys map-invert (1.11) (clojure.core/) update-keys update-vals GitHub: Medley

'Change' assoc assoc-in dissoc merge merge-with select-keys update update-in (clojure.set/) rename-keys map-invert (1.11) (clojure.core/) update-keys update-vals GitHub: Medley

[jafingerhut]

I know that update-keys and update-vals are now in the clojure.core namespace. There are still potentially useful functions that operate on maps in Medley that are not in clojure.core. Do you think the value of folks looking at the cheatsheet knowing about the existence of Medley is too low to keep it around now?