fsm: log tweak: upon free, say 'Freeing instance'

Logging 'Release' is a bit ambiguous. At first I tought a subscriber
connection was being released, IMHO 'Freeing instance' better describes that
we are freeing an osmo_fsm_inst.

Change-Id: I5cf99707d2ba5620b2988f777fa39cc806ec0212
diff --git a/src/fsm.c b/src/fsm.c
index 842c766..319c842 100644
--- a/src/fsm.c
+++ b/src/fsm.c
@@ -467,7 +467,7 @@
 	if (fi->fsm->cleanup)
 		fi->fsm->cleanup(fi, cause);
 
-	LOGPFSMSRC(fi, file, line, "Release\n");
+	LOGPFSMSRC(fi, file, line, "Freeing instance\n");
 	osmo_fsm_inst_free(fi);
 
 	/* indicate our termination to the parent */