Fix URL to truseq document

The original [link][orig] to "Illumina TruSeq Adapters De-Mystified" has been expired. 

It can be replaced with the [link][new] still hosted by the same institution (TUCF-Genomics).

[orig]: http://genomics.med.tufts.edu/documents/protocols/TUCF_Understanding_Illumina_TruSeq_Adapters.pdf
[new]: http://tucf-genomics.tufts.edu/documents/protocols/TUCF_Understanding_Illumina_TruSeq_Adapters.pdf