[docs] fix typo in xformers_mingpt.ipynb

[eltociear]

What does this PR do?

avaiable -> available

Before submitting

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• Did you read the contributor guideline?
• Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
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• Did you make sure to update the docs?
  • N/A
• Did you write any new necessary tests?
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• Did you update the changelog? (if needed)
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PR review

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[codecov-commenter]

Codecov Report

Base: 89.89% // Head: 89.89% // No change to project coverage 👍

Coverage data is based on head (87099cb) compared to base (bc08bbc).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #657   +/-   ##
=======================================
  Coverage   89.89%   89.89%           
=======================================
  Files          88       88           
  Lines        5382     5382           
=======================================
  Hits         4838     4838           
  Misses        544      544           

Flag	Coverage Δ
Python	89.89% <ø> (ø)

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[blefaudeux]

thanks !