Add Groovy DSL example for jvmTarget to homepage (#2423)
Add Groovy DSL example for jvmTarget to homepage (#2423)

This complies with the source code in the README.

follow-up task to #2422
arturbosch/detekt,arturbosch/detekt,rock3r/detekt,rock3r/detekt,arturbosch/detekt,rock3r/detekt