crypto/md5: fix comment typo.

Fixes #210.

R=rsc
CC=golang-dev
https://golang.org/cl/155057

src/pkg/crypto/md5/md5.go

@@ -38,7 +38,7 @@ func (d *digest) Reset() {
 	d.len = 0;
 }
 
-// New returns a hash.Hash computing the SHA1 checksum.
+// New returns a new hash.Hash computing the MD5 checksum.
 func New() hash.Hash {
 	d := new(digest);
 	d.Reset();