Fix wrong Groovy DSL example (#2302)

File#resolve(String) doen't exist in Groovy

docs/src/doc/docs/user_guide/gradle/usage.md

@@ -45,18 +45,18 @@ If you encounter any problems when migrating from older versions of Dokka, pleas
 
 Minimal configuration (with custom output directory only):
 
-Kotlin
+Kotlin DSL
 ```kotlin
 tasks.dokkaHtml.configure {
     outputDirectory.set(buildDir.resolve("dokka"))
 }
 ```
 
 
-Groovy
-```kotlin
-tasks.named("dokkaHtml") {
-    outputDirectory.set(buildDir.resolve("dokka"))
+Groovy DSL
+```groovy
+dokkaHtml {
+    outputDirectory.set(file("${buildDir}/dokka"))
 }
 ```
 
@@ -370,4 +370,4 @@ tasks.dokkaHtmlPartial.configure {
 
 Please see the [Dokka Gradle single module example project](https://github.com/Kotlin/dokka/tree/master/examples/gradle/dokka-gradle-example) or [multimodule](https://github.com/Kotlin/dokka/tree/master/examples/gradle/dokka-multimodule-example) for an example.
 
-Also see [generated documentation](https://Kotlin.github.io/dokka/examples/dokka-gradle-example/html) in `HTML` format.
+Also see [generated documentation](https://Kotlin.github.io/dokka/examples/dokka-gradle-example/html) in `HTML` format.