Added format option for AiPath action

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #45
| License       | MIT
| Doc PR        | symfony-cmf/symfony-cmf-docs#353

As the AiPath action always suffixes, I decided to remove the `%s` from the format option.

Please also note that using a prefix with a slash is not support.