blake2s,blake2b: fix typo in package doc

Change-Id: I6c33e69dc3e45d86843861973952d6f11c0fae88
Reviewed-on: https://go-review.googlesource.com/31397
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

blake2s/blake2s.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package blake2s implemnets the BLAKE2s hash algorithm as
+// Package blake2s implements the BLAKE2s hash algorithm as
 // defined in RFC 7693.
 package blake2s