Add on_attester_slashing() to handlers list

specs/phase0/fork-choice.md

@@ -48,6 +48,7 @@ The head block root associated with a `store` is defined as `get_head(store)`. A
 - `on_tick(store, time)` whenever `time > store.time` where `time` is the current Unix time
 - `on_block(store, block)` whenever a block `block: SignedBeaconBlock` is received
 - `on_attestation(store, attestation)` whenever an attestation `attestation` is received
+- `on_attester_slashing(store, attester_slashing)` whenever an attester slashing `attester_slashing` is received
 
 Any of the above handlers that trigger an unhandled exception (e.g. a failed assert or an out-of-range list access) are considered invalid. Invalid calls to handlers must not modify `store`.
 
@@ -485,4 +486,4 @@ def on_attester_slashing(store: Store, attester_slashing: AttesterSlashing) -> N
     indices = set(attestation_1.attesting_indices).intersection(attestation_2.attesting_indices)
     for index in indices:
         store.equivocating_indices.add(index)
-```
+```