fsm: fix apidoc of osmo_fsm_state_name()

The api documentation of osmo_fsm_state_name() refers to an FSM
instance, but it really means the state of an FSM.

Change-Id: I88ddd6048426d380c49170e66f57b3843398c046
diff --git a/src/fsm.c b/src/fsm.c
index b5af2e7..1f6141fa 100644
--- a/src/fsm.c
+++ b/src/fsm.c
@@ -414,7 +414,7 @@
 		return fi->fsm->name;
 }
 
-/*! get human-readable name of FSM instance
+/*! get human-readable name of FSM state
  *  \param[in] fsm FSM descriptor
  *  \param[in] state FSM state number
  *  \returns string rendering of the FSM state