[io grant] Change return value for mkdir

Per rakudo/rakudo@c01ebea0a0

Author: zoffixznet

doc/Type/IO/Path.pod6

@@ -518,11 +518,12 @@ only C<:d> is set to C<True>.
 
 =head2 routine mkdir
 
-    method mkdir(IO::Path:D: Int() $mode = 0o777 --> Bool:D)
-    sub mkdir(   IO() $path, Int() $mode = 0o777 --> Bool:D)
+    method mkdir(IO::Path:D: Int() $mode = 0o777 --> IO::Path:D)
+    sub mkdir(   IO() $path, Int() $mode = 0o777 --> IO::Path:D)
 
 Creates a new directory. See L«C<mode>|/routine/mode» for explanation and
-valid values for C<$mode>. Returns C<True> on success;
+valid values for C<$mode>. Returns the L<IO::Path> object pointing to
+the newly created directory on success;
 L<fails|/routine/fail> with L<X::IO::Mkdir> if directory cannot be created.
 
 Creates parent directories, as needed. That is, C<mkdir "foo/bar"> will