fix figcaption a11y warning

[Rich-Harris]

fixes #991

[codecov-io]

Codecov Report

Merging #996 into master will increase coverage by <.01%.
The diff coverage is 83.33%.

@@           Coverage Diff            @@
##           master   #996      +/-   ##
========================================
+ Coverage   91.99%    92%   +<.01%     
========================================
  Files         115    115              
  Lines        4271   4275       +4     
  Branches     1359   1361       +2     
========================================
+ Hits         3929   3933       +4     
  Misses        148    148              
  Partials      194    194

Impacted Files	Coverage Δ
src/validate/html/a11y.ts	95.18% <83.33%> (+0.24%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a8eaa7e...9fce7a1. Read the comment docs.