Fix typo in ecmult_const_impl.h

Summary:
Fix small typo in the reference given for the wNAF method

This is a backport fo libsecp256k1 [[bitcoin-core/secp256k1#742 | PR742]]

Test Plan:
  ninja check-secp256k1

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7585

src/ecmult_const_impl.h

@@ -46,7 +46,7 @@
  *
  *  Adapted from `The Width-w NAF Method Provides Small Memory and Fast Elliptic Scalar
  *  Multiplications Secure against Side Channel Attacks`, Okeya and Tagaki. M. Joye (Ed.)
- *  CT-RSA 2003, LNCS 2612, pp. 328-443, 2003. Springer-Verlagy Berlin Heidelberg 2003
+ *  CT-RSA 2003, LNCS 2612, pp. 328-443, 2003. Springer-Verlag Berlin Heidelberg 2003
  *
  *  Numbers reference steps of `Algorithm SPA-resistant Width-w NAF with Odd Scalar` on pp. 335
  */