Add keyword maven-target to help of maven plugin

The current help vor maven plugin doesn't include the description for maven-targets.
This description should be included.

Current output for snapcraft help maven (snapcraft version 2.23):
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This plugin is useful for building parts that use maven.

The maven build system is commonly used to build Java projects.
The plugin requires a pom.xml in the root of the source tree.

This plugin uses the common plugin keywords as well as those for "sources".
For more information check the 'plugins' topic for the former and the
'sources' topic for the latter.

Additionally, this plugin uses the following plugin-specific keywords:

    - maven-options:
      (list of strings)
      flags to pass to the build using the maven semantics for parameters.
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