eip7732: add PTC subsection to validator assignment section (#4713)

Author: jihoonsong

specs/gloas/validator.md

@@ -8,6 +8,7 @@
 - [Configuration](#configuration)
   - [Time parameters](#time-parameters)
 - [Validator assignment](#validator-assignment)
+  - [Payload timeliness committee](#payload-timeliness-committee)
   - [Lookahead](#lookahead)
 - [Beacon chain responsibilities](#beacon-chain-responsibilities)
   - [Attestation](#attestation)
@@ -44,6 +45,8 @@ validator" to implement Gloas.
 
 ## Validator assignment
 
+### Payload timeliness committee
+
 A validator may be a member of the new Payload Timeliness Committee (PTC) for a
 given slot. To check for PTC assignments, use
 `get_ptc_assignment(state, epoch, validator_index)` where `epoch <= next_epoch`,