Clarify --engine option in --help string

[mbravenboer]

The engine option says 'default engine'. If the engine name is not correct than it (correctly imo) gives an error, so I think the option description should be something like 'engine to use'.

$ rai exec --help
Execute a transaction on the given database

Usage:
  rai exec database [flags]

Flags:
  -c, --code string     rel source code
  -e, --engine string   default engine

[billscheidel-rai]

Note: This issue has been migrated to https://relationalai.atlassian.net/browse/RAI-5298.

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