README: replace unicode character

Only use ASCII characters in README.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

README

@@ -5,7 +5,7 @@ It is described in the following paper:
 
 Daniel J. Bernstein, Niels Duif, Tanja Lange, Peter Schwabe, Bo-Yin Yang
 High-speed high-security signatures
-Journal of Cryptographic Engineering 2 (2012), 77–89
+Journal of Cryptographic Engineering 2 (2012), 77-89
 2011-09-26
 http://ed25519.cr.yp.to/ed25519-20110926.pdf