crypto/md5,crypto/sha512: fix AppendBinary -> BinaryAppender docs typo

crypto/md5,crypto/sha512: fix AppendBinary -> BinaryAppender docs typo

Change-Id: Ia2caecd0a54e1100ad523f0dc76f17babcd0ffca
Reviewed-on: https://go-review.googlesource.com/c/go/+/626878
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>