Add a comment to Halstead Rust test

[Luni-4]

tree-sitter-rust does not parse the comma in macros, but the ! operator instead

[codecov-io]

Codecov Report

Merging #575 (bfeb836) into master (02faac5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #575   +/-   ##
=======================================
  Coverage   27.58%   27.58%           
=======================================
  Files          45       45           
  Lines        5684     5684           
  Branches      857      857           
=======================================
  Hits         1568     1568           
  Misses       3563     3563           
  Partials      553      553           

Impacted Files	Coverage Δ
src/metrics/halstead.rs	59.66% <ø> (ø)

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