Fix property in Mojo Javadoc tag

${aSystemProperty} doesn't work with property attribute.

https://maven.apache.org/developers/mojo-api-specification.html

maven-plugin-plugin 2.x:
@parameter expression="${aSystemProperty}" default-value="${anExpression}"

maven-plugin-plugin 3.x:
@parameter property="aSystemProperty" default-value="${anExpression}"
Merge pull request #174 from shirosaki/fix_mojo_config

Fix property in Mojo Javadoc tag