Typo in 01_foundations/Code.ipynb

In the first plot of the `Square` and `ReLU` functions:

``` Python
ax[0].set_title("Square function")
ax[0].set_xlabel("input")
ax[0].set_ylabel("input")
```
`ylabel` is a duplicate of `xlabel`.

should have been:

``` Python
ax[0].set_title("Square function")
ax[0].set_xlabel("input")
ax[0].set_ylabel("output")
```