*: 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/default_error_retval.c b/libbb/default_error_retval.c
index 0b19f21..4da5b3e 100644
--- a/libbb/default_error_retval.c
+++ b/libbb/default_error_retval.c
@@ -2,7 +2,7 @@
 /*
  * Copyright (C) 2003  Manuel Novoa III  <mjn3@codepoet.org>
  *
- * 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.
  */
 
 /* Seems silly to copyright a global variable.  ;-)  Oh well.