Fix dead link to Schnorr spec

Summary: bitcoincash.org no longer hosts specifications

Test Plan:
Check that old link is dead.
Check that link works at https://github.com/Mengerian/bitcoin-abc/tree/secp-spec/src/secp256k1
Grep for other instances of spec links to bitcoincash.org - none found.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9730