Add a constant for slash penalty multiplier

Make the multipler on slashed penalties (if portion `p` of validators are slashed each one loses `k*p` an explicit constant).

specs/phase0/beacon-chain.md

@@ -195,6 +195,7 @@ The following values are (non-configurable) constants used throughout the specif
 | `HYSTERESIS_QUOTIENT` | `uint64(4)` |
 | `HYSTERESIS_DOWNWARD_MULTIPLIER` | `uint64(1)` |
 | `HYSTERESIS_UPWARD_MULTIPLIER` | `uint64(5)` |
+| `PROPORTIONAL_SLASHING_MULTIPLIER` | `uint64(3)` |
 
 - For the safety of committees, `TARGET_COMMITTEE_SIZE` exceeds [the recommended minimum committee size of 111](http://web.archive.org/web/20190504131341/https://vitalik.ca/files/Ithaca201807_Sharding.pdf); with sufficient active validators (at least `SLOTS_PER_EPOCH * TARGET_COMMITTEE_SIZE`), the shuffling algorithm ensures committee sizes of at least `TARGET_COMMITTEE_SIZE`. (Unbiasable randomness with a Verifiable Delay Function (VDF) will improve committee robustness and lower the safe minimum committee size.)
 
@@ -1557,7 +1558,7 @@ def process_slashings(state: BeaconState) -> None:
     for index, validator in enumerate(state.validators):
         if validator.slashed and epoch + EPOCHS_PER_SLASHINGS_VECTOR // 2 == validator.withdrawable_epoch:
             increment = EFFECTIVE_BALANCE_INCREMENT  # Factored out from penalty numerator to avoid uint64 overflow
-            penalty_numerator = validator.effective_balance // increment * min(sum(state.slashings) * 3, total_balance)
+            penalty_numerator = validator.effective_balance // increment * min(sum(state.slashings) * PROPORTIONAL_SLASHING_MULTIPLIER, total_balance)
             penalty = penalty_numerator // total_balance * increment
             decrease_balance(state, ValidatorIndex(index), penalty)
 ```