cosmetic: logging.h: fix comment s/levels/subsystems

Change-Id: I242a4a44649bc4dac055985ba8fd63b2f784ee6d
diff --git a/include/osmocom/core/logging.h b/include/osmocom/core/logging.h
index 75650ba..79eec10 100644
--- a/include/osmocom/core/logging.h
+++ b/include/osmocom/core/logging.h
@@ -105,7 +105,7 @@
 #define LOGL_ERROR	7	/*!< error condition, requires user action */
 #define LOGL_FATAL	8	/*!< fatal, program aborted */
 
-/* logging levels defined by the library itself */
+/* logging subsystems defined by the library itself */
 #define DLGLOBAL	-1	/*!< global logging */
 #define DLLAPD		-2	/*!< LAPD implementation */
 #define DLINP		-3	/*!< (A-bis) Input sub-system */