fix syncCommitteeDuties log in Slot end message (#5416)

In `Slot end`, sync duties are reported one slot too late,
because sync committee duties are determined by the next slot
instead of the current one. Address that by taking account of this.

beacon_chain/nimbus_beacon_node.nim

@@ -1281,15 +1281,18 @@ proc onSlotEnd(node: BeaconNode, slot: Slot) {.async.} =
       toGaugeValue(x)
 
   template formatSyncCommitteeStatus(): string =
-    let slotsToNextSyncCommitteePeriod =
-      SLOTS_PER_SYNC_COMMITTEE_PERIOD - since_sync_committee_period_start(slot)
+    let
+      syncCommitteeSlot = slot + 1
+      slotsToNextSyncCommitteePeriod =
+        SLOTS_PER_SYNC_COMMITTEE_PERIOD -
+        since_sync_committee_period_start(syncCommitteeSlot)
 
     # int64 conversion is safe
     doAssert slotsToNextSyncCommitteePeriod <= SLOTS_PER_SYNC_COMMITTEE_PERIOD
 
-    if not node.getCurrentSyncCommiteeSubnets(slot.epoch).isZeros:
+    if not node.getCurrentSyncCommiteeSubnets(syncCommitteeSlot.epoch).isZeros:
       "current"
-    elif not node.getNextSyncCommitteeSubnets(slot.epoch).isZeros:
+    elif not node.getNextSyncCommitteeSubnets(syncCommitteeSlot.epoch).isZeros:
       "in " & toTimeLeftString(
         SECONDS_PER_SLOT.int64.seconds * slotsToNextSyncCommitteePeriod.int64)
     else: