Fix typo, rephrase access_token doc

[crgk]

Spotted a typo on the README, and gave the ReadTheDocs pages a pass to justify the commit :)

If I overstepped on editing the access_token doc, lemme know. Seemed like some OAuth2-specific phrasing may have slipped in accidentally, where the rest doesn't refer to that protocol.

And now I'm doing this as the right user to get those sweet GitHub points.

[codecov-io]

Codecov Report

Merging #89 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #89   +/-   ##
=======================================
  Coverage   93.58%   93.58%           
=======================================
  Files          13       13           
  Lines         998      998           
=======================================
  Hits          934      934           
  Misses         64       64

Impacted Files	Coverage Δ
jose/jwt.py	94.19% <ø> (ø)	⬆️

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[mpdavis]

lgtm