*: make GNU licensing statement forms more regular

This change retains "or later" state! No licensing _changes_ here,
only form is adjusted (article, space between "GPL" and "v2" and so on).

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/coreutils/df.c b/coreutils/df.c
index 5eeb5b4..af9b77b 100644
--- a/coreutils/df.c
+++ b/coreutils/df.c
@@ -5,7 +5,7 @@
  * Copyright (C) 1999-2004 by Erik Andersen <andersen@codepoet.org>
  * based on original code by (I think) Bruce Perens <bruce@pixar.com>.
  *
- * Licensed under GPLv2 or later, see file LICENSE in this tarball for details.
+ * Licensed under GPLv2 or later, see file LICENSE in this source tree.
  */
 
 /* BB_AUDIT SUSv3 _NOT_ compliant -- option -t missing. */