Update README.md

Author: jaehyukchoi

README.md

@@ -38,7 +38,7 @@ array([15.71361973,  9.69250803,  5.52948546,  2.94558338,  1.48139131])
 
 `In [2]:`
 ```python
-sigma = np.array([0.2], [0.5])
+sigma = np.array([[0.2], [0.5]])
 m = pf.Bsm(sigma, intr=0.05, divr=0.1) # sigma in axis=0
 m.price(strike=np.array([90, 95, 100]), spot=100, texp=1.2, cp=np.array([-1,1,1]))
 ```