Fix typo

Author: birgits

windpowerlib/wind_speed.py

@@ -152,7 +152,7 @@ def hellman(
 
     For the Hellman exponent :math:`\alpha` many studies use a value of 1/7 for
     onshore and a value of 1/9 for offshore. The Hellman exponent can also
-    be calulated by the following equation [2]_ [3]_:
+    be calculated by the following equation [2]_ [3]_:
 
     .. math:: \alpha=\frac{1}{\ln\left(\frac{h_{hub}}{z_0} \right)}