docs: exclude migrate-archive during chain upgrade

Folks in the validator community reported frequent failures of the
`--migrate-archive` flag. I was able to use it during upgrades,
and posted the results on snapshots.penumbra.zone, but it's not actually
required for the chain upgrade, so let's remove it in order to keep the
process on rails.

docs/guide/src/node/pd/chain-upgrade.md

@@ -61,7 +61,7 @@ An example log message emitted by `pd migrate` without providing `--genesis-star
 The value after `now=` is what should be copied. In practice, for testnets, Penumbra Labs will advise on a genesis time
 and provide that value in the documentation. Or should we just pick a genesis start ahead of time, and use that for all?
 -->
-5. Apply the migration: `pd migrate --genesis-start "{{ #include ../../upgrade_genesis_time_71_72.md }}" --target-directory ~/.penumbra/testnet_data/node0/pd-exported-state-71/ --migrate-archive ~/.penumbra/testnet_data/node0/pd-migrated-state-{{ #include ../../penumbra_version.md }}.tar.gz`.
+5. Apply the migration: `pd migrate --genesis-start "{{ #include ../../upgrade_genesis_time_71_72.md }}" --target-directory ~/.penumbra/testnet_data/node0/pd-exported-state-71/`
    You must use that precise genesis time, otherwise your node will not be able to reach consensus with the rest of the network.
 6. Move the migrated state into place: `rm -r ~/.penumbra/testnet_data/node0/pd/rocksdb && mv ~/.penumbra/testnet_data/node0/pd-exported-state-71/rocksdb ~/.penumbra/testnet_data/node0/pd/`
 7. Copy the new genesis into place: `cp ~/.penumbra/testnet_data/node0/pd-exported-state-71/genesis.json ~/.penumbra/testnet_data/node0/cometbft/config/genesis.json`