typo fix: PCKE -> PKCE

[julianfoad]

Just a tiny typo fix: 'PKCE' was mis-spelled 'PCKE' in one place.

$ grep -ri pcke .
README.md:- Proof Key for Code Exchange (PCKE) by [...]

Feel free to fix manually, or pull it from my (not github) fork, as you prefer:

git pull https://lab.trax.im/fork/github/zmartzone/liboauth2.git typo-1

Thanks!

[julianfoad]

Thanks!