Correct docstring of v_wind_hellman

Author: SabineHaas

windpowerlib/wind_speed.py

@@ -101,7 +101,7 @@ def v_wind_hellman(v_wind, v_wind_height, hub_height, hellman_exp=None,
     hellman_exp : float
         The Hellman exponent, which combines the increase in wind speed due to
         stability of atmospheric conditions and surface roughness into one
-        constant. Default: 1/7. If roughness length is given
+        constant. Default: None. If None and roughness length is given
         `hellman_exp` = 1 / ln(h_hub/z_0).
     z_0 : pandas.Series or array or float
         Roughness length. Default: None.