crypto/sha512: update comment for the Sum512 function.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/10660043
diff --git a/src/pkg/crypto/sha512/sha512.go b/src/pkg/crypto/sha512/sha512.go
index 6825c22..d2ada51 100644
--- a/src/pkg/crypto/sha512/sha512.go
+++ b/src/pkg/crypto/sha512/sha512.go
@@ -184,7 +184,7 @@
 	return digest
 }
 
-// Sum returns the SHA512 checksum of the data.
+// Sum512 returns the SHA512 checksum of the data.
 func Sum512(data []byte) [Size]byte {
 	var d digest
 	d.Reset()