Converter generator XSLT: fix reference to attribute, _key-name

[galtm]

Committer Notes

Closes #82

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[github-actions]

XSpec Test Results

  2 files  ±0  40 suites  ±0   0s ⏱️ ±0s
105 tests ±0  90 ✅ ±0  15 💤 ±0  0 ❌ ±0 
114 runs  ±0  99 ✅ ±0  15 💤 ±0  0 ❌ ±0 

Results for commit 2949722. ± Comparison against base commit 0010340.

[galtm]

I rebased my commit off the develop branch and force-pushed, to refresh this pull request. The checks ran and all passed.