Fix RPC help for dumptxoutset
Fix RPC help for dumptxoutset

Summary: A mandatory argument cannot have a default value. The help message was not formatted correctly, as a result.

Test Plan:
Before:
```
$ src/bitcoin-cli help dumptxoutset
dumptxoutset path to the output file. If relative, will be prefixed by datadir.

Write the serialized UTXO set to disk.

Arguments:
1. path    (string, required)

Result:
...
```

After:
```
$ src/bitcoin-cli help dumptxoutset
dumptxoutset "path"

Write the serialized UTXO set to disk.

Arguments:
1. path    (string, required) path to the output file. If relative, will be prefixed by datadir.

Result:
...
```

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10155

Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc