warning: fixed alt_diff type

Author: tridge

MAVProxy/modules/mavproxy_warning.py

@@ -19,7 +19,7 @@ def __init__(self, mpstate):
               ('details', bool, True),
               ('min_sat', int, 10),
               ('sat_diff', int, 5),
-              ('alt_diff', int, 20),
+              ('alt_diff', float, 20),
               ('airspd_diff', float, 5),
               ('airspd_age', float, 5),
               ('esc_group1', int, 0),