Fix typo in the documentation of the API of libsecp256k1
Fix typo in the documentation of the API of libsecp256k1

Summary: Backport of PR#458 by Tim Ruffing

Test Plan:
  make check

Reviewers: schancel, MattBitprim, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D781

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