Fix broken link for network cookbook tutorial

[erictleung]

Link formerly linked to author's homepage rather than the resource itself. Although the author notes that this tutorial is out of date, this can be kept as a good enough resource to start with network analysis.

[oscarbaruffa]

thanks Eric! Hmm maybe I should remove it - don't want bbofR to host too many out of date things but if you think its still a good resource i might keep it for a while