Fix an error when logging a warning message

src/pypfilt/state.py

@@ -171,7 +171,7 @@ def earlier_states(hist, ix, steps):
     # Don't go too far back (negative indices jump into the future).
     if steps > ix:
         msg_fmt = 'Cannot look back {} time-steps, will look back {}'
-        logger.warn(msg_ftm.format(steps, ix))
+        logger.warn(msg_fmt.format(steps, ix))
     steps = min(steps, ix)
 
     # Start with the parent indices for the current particles, which allow us