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}"
```

Fix issue 860:
https://code.google.com/p/wro4j/issues/detail?id=860