ENH: fixing variable name

Author: ArthurJWH

rocketpy/rocket/parachute.py

@@ -198,9 +198,9 @@ def __init__(
         self.porosity = porosity
         self.ka = 1.068 * (
             1
-            - 1.465 * self.parachute_porosity
-            - 0.25975 * self.parachute_porosity**2
-            + 1.2626 * self.parachute_porosity**3
+            - 1.465 * self.porosity
+            - 0.25975 * self.porosity**2
+            + 1.2626 * self.porosity**3
         )
 
         alpha, beta = self.noise_corr