nav reducer: Pin down a type fixme more narrowly.
nav reducer: Pin down a type fixme more narrowly.

This makes more explicit exactly what the type mismatch is that
this $FlowFixMe is marking, which will make it more robust to the
refactoring we're about to do.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile