Spelling errors in retry.py docs

[JunchengL]

Hi,

I have found the following spelling errors in retry.py docs while I am reading the code:

        initial (float): The minimum a,out of time to delay in seconds. This
            must be greater than 0.
        maximum (float): The maximum amout of time to delay in seconds.

which should be:

        initial (float): The minimum amount of time to delay in seconds. This
            must be greater than 0.
        maximum (float): The maximum amount of time to delay in seconds.

I will open a PR to fix it.