fix typo in doc

Author: robstoll

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

@@ -60,7 +60,7 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
   class NonEmptyFunction(args: List[Tree], body: Tree, val mods: Modifiers) extends Function(args, body)
 
   /** A function created from a wildcard expression
-   *  @param  placeHolderParams  a list of definitions of synthetic parameters
+   *  @param  placeholderParams  a list of definitions of synthetic parameters.
    *  @param  body               the function body where wildcards are replaced by
    *                             references to synthetic parameters.
    */