Mistake in Beale function

It should read
```python
res = (1.5  - x + x*y) **2 + (2.25 - x + x*y**2)**2 + (2.625 - x + x*y**3)**2
```
instead of
```python
res = (1.5  - x + x*y) **2 + (2.25 - x + x*y**2)**2 + (2.625 - x + x*y**3)*2
```
(typo in the exponent)