adjusted/fixed maven-scala-plugin maven repo URL

according to scala-tools.org: scala-tools.org is in no longer serving jars. Please change your configurations to point to Sonatype's Maven repository at https://oss.sonatype.org/content/groups/scala-tools; only maven-scala-plugin version 2.15.2 is available (but no maven-scala-plugin-2.15.2-SNAPSHOT)