Fix DottyPlugin for Scala3 check

Author: BarkingBad

sbt-dotty/sbt-test/sbt-dotty/scaladoc/build.sbt

@@ -1,7 +1,7 @@
 scalaVersion := sys.props("plugin.scalaVersion")
 
 TaskKey[Unit]("assertUsingScaladoc") := {
-  assert(useScaladoc.value)
+  assert(useScaladoc.value, "Scaladoc version does not match the expected pattern")
 }
 
 TaskKey[Unit]("checkScaladocOptions") := {

sbt-dotty/src/dotty/tools/sbtplugin/DottyPlugin.scala

@@ -374,7 +374,7 @@ object DottyPlugin extends AutoPlugin {
       resolvers ++= (if(!useScaladoc.value) Nil else Seq(Resolver.jcenterRepo)),
       useScaladoc := {
         val v = scalaVersion.value
-        v.startsWith("3.0.0") && !v.startsWith("3.0.0-M1") && !v.startsWith("3.0.0-M2")
+        v.startsWith("3.") && !v.startsWith("3.0.0-M1") && !v.startsWith("3.0.0-M2")
       },
       // We need to add doctool classes to the classpath so they can be called
       doc / scalaInstance := Def.taskDyn {