spec: fix shuffle direction in get_crosslink_committees_at_slot

eth2/types/src/beacon_state/epoch_cache.rs

@@ -288,7 +288,7 @@ impl EpochCrosslinkCommitteesBuilder {
                 self.active_validator_indices,
                 spec.shuffle_round_count,
                 &self.shuffling_seed[..],
-                true,
+                false,
             )
             .ok_or_else(|| Error::UnableToShuffle)?
         };

eth2/types/src/beacon_state/epoch_cache/tests.rs

@@ -27,7 +27,7 @@ fn do_sane_cache_test(
         active_indices,
         spec.shuffle_round_count,
         &expected_seed[..],
-        true,
+        false,
     )
     .unwrap();
 

eth2/utils/swap_or_not_shuffle/src/shuffle_list.rs

@@ -18,6 +18,8 @@ const TOTAL_SIZE: usize = SEED_SIZE + ROUND_SIZE + POSITION_WINDOW_SIZE;
 /// Credits to [@protolambda](https://github.com/protolambda) for defining this algorithm.
 ///
 /// Shuffles if `forwards == true`, otherwise un-shuffles.
+/// It holds that: shuffle_list(shuffle_list(l, r, s, true), r, s, false) == l
+///           and: shuffle_list(shuffle_list(l, r, s, false), r, s, true) == l
 ///
 /// Returns `None` under any of the following conditions:
 ///  - `list_size == 0`