Doc change the default value of pymc.draw from 500 to 1.

![image](https://user-images.githubusercontent.com/42216008/156758069-b44af532-08dd-49ca-af45-e37ab1bc1d56.png)


In [pymc.draw](https://docs.pymc.io/en/latest/api/generated/pymc.draw.html#pymc.draw), the function takes the dafault value of draws=1 but in docstring it's mentioned the default value as 500.