Swap an example for keep

[sogaiu]

This PR swaps an example of keep.

The removal of one example is motivated by some comments starting here.

Motivation for the additional example includes the points:

1. Use of a symbol in the predicate argument position that represents a pre-existing function instead of using a function constructed via the use of fn (as there are already examples of the latter)
2. Showing a use of keep which is in some sense about as simple as things might get

[rwtolbert]

LGTM

[sogaiu]

Thanks for taking a look!