statcast_pitcher_spin.md has a typo listing the wrong function.

The doc currently shows:
```
statcast_pitcher(start_dt=[yesterday's date], end_dt=None, player_id)

```

When the doc should show the correct function being described:
```
statcast_pitcher_spin(start_dt=[yesterday's date], end_dt=None, player_id)

```
This function name matches the example. 
```
from pybaseball import statcast_pitcher_spin
```