Fix typo in docs for _context_set_illegal_callback
Fix typo in docs for _context_set_illegal_callback

Summary: This is a backport of secp256k1 PR656

Test Plan:
  ninja check-secp256k1

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5354

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