Fix snapshot docs URL, currently only appears on snapshot docs

Author: jamesfredley

grails-gradle/docs-core/src/main/groovy/grails/doc/dropdown/CreateReleaseDropDownTask.groovy

@@ -154,7 +154,7 @@ abstract class CreateReleaseDropDownTask extends DefaultTask {
  softwareVersions
  .forEach { softwareVersion ->
  final String versionName = softwareVersion?.versionText
- final String href = GRAILS_DOC_BASE_URL + "/" + versionName + "/" + pageRelativePath
+ final String href = GRAILS_DOC_BASE_URL + "/" + (versionName?.endsWith("-SNAPSHOT") ? 'snapshot' : versionName) + "/" + pageRelativePath
  options << option(href, versionName, version == versionName)
  }