Modify tanh code for README

README.md

@@ -17,8 +17,7 @@ Example use:
 >>> from autograd import grad    # The only autograd function you may ever need
 >>>
 >>> def tanh(x):                 # Define a function
-...     y = np.exp(-2.0 * x)
-...     return (1.0 - y) / (1.0 + y)
+...     return (1.0 - np.exp(-x))  / (1.0 + np.exp(-x))
 ...
 >>> grad_tanh = grad(tanh)       # Obtain its gradient function
 >>> grad_tanh(1.0)               # Evaluate the gradient at x = 1.0