contrib: update dumptxoutset command in utxo_to_sqlite doc

Running the `dumptxoutset` command without a `type` parameter leads
to the following error. Update the tool documentation to make it
easier to follow.

`Invalid snapshot type "" specified. Please specify "rollback" or "latest"`

Author: rkrux

contrib/utxo-tools/utxo_to_sqlite.py

@@ -5,7 +5,7 @@
 """Tool to convert a compact-serialized UTXO set to a SQLite3 database.
 
 The input UTXO set can be generated by Bitcoin Core with the `dumptxoutset` RPC:
-$ bitcoin-cli dumptxoutset ~/utxos.dat
+$ bitcoin-cli dumptxoutset ~/utxos.dat latest
 
 The created database contains a table `utxos` with the following schema:
 (txid TEXT, vout INT, value INT, coinbase INT, height INT, scriptpubkey TEXT)