Fix docstring for removed arg (browser_profile)

[mdmintz]

browser_profile was removed from start_session() args in remote/webdriver.py, but it's still in the docstring.
It should be removed from the docstring to match the remaining args.

Types of changes

• Fix docstring

Checklist

• I have read the contributing document.
• My change is to the documentation / docstrings.
• I have updated the documentation accordingly.

[titusfortner]

Thanks!