Fix TypeComparer.explained usage in TreeChecker

Author: smarter

compiler/src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -315,7 +315,7 @@ class TreeChecker extends Phase with SymTransformer {
                |Original tree : ${tree.show}
                |After checking: ${tree1.show}
                |Why different :
-             """.stripMargin + core.TypeComparer.explained((tp1 <:< tp2)(_))
+             """.stripMargin + core.TypeComparer.explained(tp1 <:< tp2)
           if (tree.hasType) // it might not be typed because Typer sometimes constructs new untyped trees and resubmits them to typedUnadapted
             assert(isSubType(tree1.tpe, tree.typeOpt), divergenceMsg(tree1.tpe, tree.typeOpt))
           tree1