cardano-testnet-test: fix a test stanza

[smelc]

Description

FIxes the command documented to launch a test

How to trust this PR

1. Try executing the previous stanza: cabal test cardano-testnet-test --test-options '-p "/DRepRetirement/"'
2. Observe it doesn't launch anything
3. Try the corrected test stanza: cabal test cardano-testnet-test --test-options '-p "/DRep Retirement/"'
4. Observer the corresponding test is executed this time

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• CI passes. See note on CI. The following CI checks are required:
• Self-reviewed the diff