Fix mistake in scala#11584

Author: rjolly

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -919,7 +919,7 @@ object desugar {
                 case ValDefs(vparam :: Nil) =>
                   if !vparam.mods.is(Given) then
                     val (leadingUsing, otherExtParamss) = ext.paramss.span(isUsingOrTypeParamClause)
-                    leadingUsing ::: params1 :: otherExtParamss ::: paramss1
+                    leadingUsing ::: params :: otherExtParamss ::: paramss1
                   else badRightAssoc("cannot start with using clause")
                 case _ =>
                   noVParam