Doc fix in iallocator.rst: multi-evac requires "evac_nodes"

The request argument for multi-evacuate mode is "evac_nodes", not "nodes"
(the example later in the file has the correct name already).

Signed-off-by: Adeodato Simo <dato@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>