token_lookup has an incorrect doc string for the accessor kwarg

[jessecooper]

https://github.com/hvac/hvac/blob/develop/hvac/v1/__init__.py#L429
correct doc sting

:param accessor:
:type accessor: bool

Kind of a silly one and I will open up a PR if time permits but caused me a bit of pain when just looking at
client.token_lookup.__doc__ because I sat there trying to plug in accessor=<token_accessor> and wondering why it was coming back with a self lookup. Then having to go directly into the code on github to see the default assigned to a bool leading me to the proper usage of this method.