doc: Fix the Doxygen section endings

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/src/gsm/a5.c b/src/gsm/a5.c
index 34f271a..356060a 100644
--- a/src/gsm/a5.c
+++ b/src/gsm/a5.c
@@ -364,4 +364,4 @@
 	}
 }
 
-/*! }@ */
+/*! @} */