fix(Tree): Invalid DOM property 'tabindex'

[sophieH29]

Fix for:

[codecov]

Codecov Report

Merging #1345 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1345   +/-   ##
=======================================
  Coverage   72.31%   72.31%           
=======================================
  Files         762      762           
  Lines        5765     5765           
  Branches     1687     1687           
=======================================
  Hits         4169     4169           
  Misses       1590     1590           
  Partials        6        6

Impacted Files	Coverage Δ
.../accessibility/Behaviors/Tree/treeTitleBehavior.ts	100% <ø> (ø)	⬆️

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