doc: mcumgr: fix mixed up occurrences of :c:enum: and :c:enumerator:

While Breathe seems to not care and creates hyperlinks either way, the
proper role to mention a C enumerator is :c:enumerator:. :c:enum: is to
be used to refer to the enum itself.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>