[doc] Provide proper name for 'auth' group

Change-Id: I5fb1c34475ea9db36c2ec5cddc988074a72a35f7
diff --git a/include/osmocom/crypt/auth.h b/include/osmocom/crypt/auth.h
index 1b017c0..4dbc6a4 100644
--- a/include/osmocom/crypt/auth.h
+++ b/include/osmocom/crypt/auth.h
@@ -1,6 +1,6 @@
 #pragma once
 
-/*! \addtogroup auth
+/*! \defgroup auth GSM/GPRS/3G Authentication
  *  @{
  * \file auth.h */