*: 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/libbb/printable_string.c b/libbb/printable_string.c
index 83a4821..a316f60 100644
--- a/libbb/printable_string.c
+++ b/libbb/printable_string.c
@@ -4,7 +4,7 @@
  *
  * Copyright (C) 2010 Denys Vlasenko
  *
- * Licensed under GPL version 2, see file LICENSE in this tarball for details.
+ * Licensed under GPLv2, see file LICENSE in this source tree.
  */
 #include "libbb.h"
 #include "unicode.h"