[doc] fix link to reference

readme.md

@@ -450,8 +450,8 @@ This library contains some part of the followings software licensed by BSD-3-Cla
 * [_High-Speed Software Implementation of the Optimal Ate Pairing over Barreto-Naehrig Curves_](http://dx.doi.org/10.1007/978-3-642-17455-1_2),
    Jean-Luc Beuchat, Jorge Enrique González Díaz, Shigeo Mitsunari, Eiji Okamoto, Francisco Rodríguez-Henríquez, Tadanori Teruya,
   Pairing 2010, ([preprint](http://eprint.iacr.org/2010/354))
-* [_Faster hashing to G2_](http://dx.doi.org/10.1007/978-3-642-28496-0_25),Laura Fuentes-Castañeda,  Edward Knapp,  Francisco Rodríguez-Henríquez,
-  SAC 2011, ([preprint](https://eprint.iacr.org/2008/530))
+* [_Faster hashing to G2_](https://link.springer.com/chapter/10.1007/978-3-642-28496-0_25),Laura Fuentes-Castañeda,  Edward Knapp,  Francisco Rodríguez-Henríquez,
+  SAC 2011, ([PDF](http://cacr.uwaterloo.ca/techreports/2011/cacr2011-26.pdf))
 * [_Skew Frobenius Map and Efficient Scalar Multiplication for Pairing–Based Cryptography_](https://www.researchgate.net/publication/221282560_Skew_Frobenius_Map_and_Efficient_Scalar_Multiplication_for_Pairing-Based_Cryptography),
 Y. Sakemi, Y. Nogami, K. Okeya, Y. Morikawa, CANS 2008.