Fix toc: attempt 2

Author: mkalinin

specs/electra/beacon-chain.md

@@ -30,9 +30,9 @@
     - [`Consolidation`](#consolidation)
     - [`SignedConsolidation`](#signedconsolidation)
     - [`PendingConsolidation`](#pendingconsolidation)
-- [Modified Containers](#modified-containers)
+  - [Modified Containers](#modified-containers)
     - [`AttesterSlashing`](#attesterslashing)
-- [Extended Containers](#extended-containers)
+  - [Extended Containers](#extended-containers)
     - [`Attestation`](#attestation)
     - [`IndexedAttestation`](#indexedattestation)
     - [`BeaconBlockBody`](#beaconblockbody)