[docs] Omit parens from hashlib.md5() in hmac

[Edward Z. Yang]

Presently we have this text at http://docs.python.org/2/library/hmac.html:

    Return a new hmac object. If msg is present, the method call update(msg)
    is made. digestmod is the digest constructor or module for the HMAC
    object to use. It defaults to the hashlib.md5() constructor.

This text is misleading, as it suggests that the usage of the
digestmod param is:

    hmac.new(secret, msg, hashlib.sha256())

which is not right.

Cheers,
Edward