Remove initval arg or fix docstring

[canyon289]

Description of your problem

The usage of initval is not clear in this method. Verify if it indeed is needed. If it is fix the docstring spelling
https://github.com/pymc-devs/pymc3/blob/main/pymc3/distributions/distribution.py#L261

[michaelosthege]

My PR removes it: #4867

Unfortunately it seems to be stalled :(
It works though.