Move license out of Doxygen comment

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/bignum_mod.h b/library/bignum_mod.h
index 9d28bc7..982a9ac 100644
--- a/library/bignum_mod.h
+++ b/library/bignum_mod.h
@@ -1,6 +1,8 @@
 /**
  *  Modular bignum functions
- *
+ */
+
+/*
  *  Copyright The Mbed TLS Contributors
  *  SPDX-License-Identifier: Apache-2.0
  *