norg.json has a duplicate key

[arodland]

The norg snippets file has the key single_footnote twice, which causes vsnip to blow up trying to load the file (because it uses vim's json_decode which handles duplicate keys by returning a completely different structure). Lua stuff I think will parse it but will only see the second instance.

The second single_footnote should probably just have its key changed to ranged_footnote or something.

[OkelleyDevelopment]

Solid idea @arodland, I've raised a PR #533 if you want to give a review