avoid "effectively final" terminology in error message

Author: olhotak

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -3433,7 +3433,7 @@ extends DeclarationMsg(IllegalUnrollPlacementID):
       if method.isLocal then
         i"$prefix because it is a local method"
       else if !method.isEffectivelyFinal then
-        i"$prefix: it is not effectively final"
+        i"$prefix because it can be overridden"
       else if isCtor && method.owner.is(Trait) then
         i"implementation restriction: $prefix"
       else if method.owner.companionClass.is(CaseClass) then