[SYCL][Doc] Fix copyright year

Signed-off-by: John Pennycook <[email protected]>

Author: Pennycook

sycl/doc/extensions/SubGroup/SYCL_INTEL_sub_group.asciidoc

@@ -31,7 +31,7 @@ This document describes an extension which introduces a new +sub_group+ class re
 
 == Notice
 
-Copyright (c) 2020 Intel Corporation.  All rights reserved.
+Copyright (c) 2019-2020 Intel Corporation.  All rights reserved.
 
 == Status