update function argument

`res = pm.find_MAP(vars=[z], fmin=optimize.fmin_l_bfgs_b)` gave me the error
```python
/usr/local/lib/python3.7/dist-packages/fastprogress/fastprogress.py:74: UserWarning: Your generator is empty.
  warn("Your generator is empty.")

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-19-1697d3cf79ec> in <module>()
      1 smoothing = 0.5
----> 2 z_val = infer_z(smoothing)
      3 
      4 plot(x, y)
      5 plot(x, z_val)

1 frames
/usr/local/lib/python3.7/dist-packages/pymc3/tuning/starting.py in find_MAP(start, vars, method, return_raw, include_transformed, progressbar, maxeval, model, *args, **kwargs)
    133 
    134     try:
--> 135         opt_result = minimize(cost_func, x0, method=method, jac=compute_gradient, *args, **kwargs)
    136         mx0 = opt_result["x"]  # r -> opt_result
    137     except (KeyboardInterrupt, StopIteration) as e:

TypeError: minimize() got an unexpected keyword argument 'fmin'

```
whereas  `res = pm.find_MAP(vars=[z], method="L-BFGS-B")` did work for me

Author: keesterbrugge

examples/gaussian_processes/GP-smoothing.ipynb

@@ -207,7 +207,7 @@
     "def infer_z(smoothing):\n",
     "    with model:\n",
     "        smoothing_param.set_value(smoothing)\n",
-    "        res = pm.find_MAP(vars=[z], fmin=optimize.fmin_l_bfgs_b)\n",
+    "        res = pm.find_MAP(vars=[z], method=\"L-BFGS-B\")\n",
     "        return res[\"z\"]"
    ]
   },