Merge #664: Remove mention of ec_privkey_export because it doesn't exist

59782c68b41e4262f003135717705990b3fdc3ae Remove mention of ec_privkey_export because it doesn't exist (Jonas Nick)

Pull request description:

  Fixes #663
  There is `ec_privkey_export_der` but it takes `0` for uncompressed and not `SECP256K1_EC_UNCOMPRESSED` (which is `2`).

ACKs for top commit:
  real-or-random:
    ACK https://github.com/bitcoin-core/secp256k1/pull/664/commits/59782c68b41e4262f003135717705990b3fdc3ae
  apoelstra:
    utACK https://github.com/bitcoin-core/secp256k1/commit/59782c68b41e4262f003135717705990b3fdc3ae

Tree-SHA512: 6167581df74264be576f921d04bb8e23e16fa3b823bac4b45299079ceee38d6c74dd14a55b7b976a2cee9bdbd74dd6e3b39c0482808c1b8e65c8c80743f113a2