Update Data Carpentry URL

[davidjb]

Data Carpentry have a new URL. This fixes the broken Lessons link on the index page due to a problem with redirections from http://data-carpentry.org/lessons.

Broken URL redirections have been reported to Data Carpentry via email.

[davidjb]

According to Tracy Teal, this URL with the hyphen has never been owned/associated with Data Carpentry and according to whois is owned by:

Registrant Name:Susanna-Assunta Sansone
Registrant Organization:University of Oxford
Registrant Street: OeRC
Registrant Street: University of Oxford

If it's not your domain, that's a great reason to make sure this is correct.