Fix Docstring typo in GradientDescent (#209)

Author: GuillermoAbadLopez

qiskit_algorithms/optimizers/gradient_descent.py

@@ -75,7 +75,7 @@ class GradientDescent(SteppableOptimizer):
 
             from qiskit_algorithms.optimizers import GradientDescent
 
-            def f(x):
+            def fun(x):
                 return (np.linalg.norm(x) - 1) ** 2
 
             initial_point = np.array([1, 0.5, -0.2])
@@ -106,7 +106,7 @@ def power_law():
 
                 return power_law()
 
-            def f(x):
+            def fun(x):
                 return (np.linalg.norm(x) - 1) ** 2
 
             def grad_f(x):