Merge bitcoin-core/secp256k1#1726: docs: fix broken link to Tromer's cache.pdf paper

489a43d1bf docs: fix broken link to eprint cache.pdf paper (VolodymyrBg)

Pull request description:

  - Updated outdated link (https://www.tau.ac.il/~tromer/papers/cache.pdf)
    to working link (https://cs-people.bu.edu/tromer/papers/cache.pdf).

ACKs for top commit:
  real-or-random:
    ACK 489a43d1bf

Tree-SHA512: 7f7dc0cbd72680025fbc75df545b36359162ca8c4ed26c4d23e9999d6ddc4a270e008718b71434ea85e7dc55cd0652a1557f1103268fd6ac5875d9861b3f3f64

src/ecmult_gen_impl.h

@@ -242,7 +242,7 @@ static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp25
              *    (https://cryptojedi.org/peter/data/chesrump-20130822.pdf) and
              *   "Cache Attacks and Countermeasures: the Case of AES", RSA 2006,
              *    by Dag Arne Osvik, Adi Shamir, and Eran Tromer
-             *    (https://www.tau.ac.il/~tromer/papers/cache.pdf)
+             *    (https://eprint.iacr.org/2005/271.pdf)
              */
             for (index = 0; index < COMB_POINTS; ++index) {
                 secp256k1_ge_storage_cmov(&adds, &secp256k1_ecmult_gen_prec_table[block][index], index == abs);