Broken ACL Anthology links

There are a few references to `anthology.aclweb.org` URLs, which seem broken.

It looks like the URL pattern has changed from:

    http://anthology.aclweb.org/W/W06/W06-2920.pdf

to:

    https://aclanthology.org/W06-2920/

or:

    https://aclanthology.org/W06-2920.pdf