Plot warnings in 1D dynamics lecture

https://python.quantecon.org/scalar_dynam.html shows errors of the form 

```
/tmp/ipykernel_44418/2261905364.py:50: UserWarning: linestyle is redundantly defined by the 'linestyle' keyword argument and the fmt string "k-" (-> linestyle='-'). The keyword argument will take precedence.
  ax.plot((x, x), (0, x), 'k-', ls='dotted')
```