bpo-37218: Change example HMAC.name, to a stronger hash

In case readers of the documentation are taking this exmaple as some
form of recommendation, change it to a hashing alogrithm without known
collision attacks.

As https://tools.ietf.org/html/rfc6151 notes "attacks on HMAC-MD5 do not
seem to indicate a practical vulnerability when used as a message
authentication code", but "for a new protocol design, a ciphersuite with
HMAC-MD5 should not be included". On this basis I think it's best not to
mention it here.

Doc/library/hmac.rst

@@ -106,7 +106,7 @@ A hash object has the following attributes:
 
 .. attribute:: HMAC.name
 
-   The canonical name of this HMAC, always lowercase, e.g. ``hmac-md5``.
+   The canonical name of this HMAC, always lowercase, e.g. ``hmac-sha256``.
 
    .. versionadded:: 3.4