message: 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
diff: @@ -258,7 +258,7 @@ SECP256K1_API void secp256k1_context_destroy(
  * - void secp256k1_default_error_callback_fn(const char* message, void* data);
  * The library can call these default handlers even before a proper callback data
  * pointer could have been set using secp256k1_context_set_illegal_callback or
  - * secp256k1_context_set_illegal_callback, e.g., when the creation of a context
  + * secp256k1_context_set_error_callback, e.g., when the creation of a context
  * fails. In this case, the corresponding default handler will be called with
  * the data pointer argument set to NULL.
  *
  