Fix typo in x25519-x86_64.pl

Reviewed-by: NTim Hudson <tjh@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6726)

crypto/ec/asm/x25519-x86_64.pl

@@ -13,7 +13,7 @@
 # details see http://www.openssl.org/~appro/cryptogams/.
 # ====================================================================
 #
-# X25519 lower-level primitives for x86_86.
+# X25519 lower-level primitives for x86_64.
 #
 # February 2018.
 #