[FIX] 404 error link for fsl dtifit

[naveau]

In the Diffusion Imaging Data section of 04-modality-specific-files/01-magnetic-resonance-imaging-data.md

When refering to FSL for the bval/bvec file format, http://fsl.fmrib.ox.ac.uk/fsl/fsl4.0/fdt/fdt_dtifit.html return a 404 not found error

This section should probably now link to https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/FDT/UserGuide?highlight=(dtifit)#DTIFIT

[sappelhoff]

tagging @yarikoptic, to perhaps boost his motivation that his PR #79 will have an impact to catch these issues early :)

[sappelhoff]

@naveau could you rebase this PR?

The merge of #293 created conflicts with your changes.

Also, are you aware of our contributor's list? You may want to add yourself, see: https://github.com/bids-standard/bids-specification/wiki/Contributors

After the conflicts are resolved I would be happy to merge this.

[yarikoptic]

#293 provided a similar fix superseding this one.
I wondered if use of https://web.archive.org should be recommended (guarantee of availability) or discouraged (possibly would lead to outdated information). With added in #293 linkchecker run availability at least will be verified and we would get early alert on URLs going stale.

[sappelhoff]

I see, thanks for the note @yarikoptic

@naveau for you this means that you can still add yourself to the contributors list - and thanks for reporting the broken link. However, a rebase is no longer necessary - we'll close this PR.

re: webarchive vs standard links --> I think that with the linkchecker in place, we actively combat broken and stale links, so webarchive could rather be used to link back to URLs that are no longer present (accessible in the Internet)