fix: brokskat velars backed, not devoiced

[drammock]

in tracking down the pairs of phonemes that don't contrast, I was looking at ɣ̥ versus x̥ in 1198 (Brokskat, from Ramaswami 1982). It turns out the problem was the raw data: dot-below was used to mark "backed" phonemes, but some of those were incorrectly digitized as devoicing rings. Now:

• ɣ x correctly have "combining minus below" instead of devoicing rings
• z̥ is corrected to be ʐ (whoever entered this grammar managed to get the ʂ correct, but not ʐ)
• the semivowel ("y" in the original) is corrected from j to ɰ (as it's clearly in the "velar" column of the phoneme table).

Here's an image for reference:

@bambooforest I'll self-merge as this seems pretty straightforward to me, but if you disagree with the solution LMK and we can revert/redo.