Fix warning of unsuppoted attribute

[blegat]

The comma doesn't do what I expected:

julia> @warn("a", "b")
┌ Warning: a
│   b = "b"
└

[codecov-io]

Codecov Report

Merging #1027 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1027      +/-   ##
==========================================
- Coverage   95.19%   95.16%   -0.04%     
==========================================
  Files         100      100              
  Lines       11279    11329      +50     
==========================================
+ Hits        10737    10781      +44     
- Misses        542      548       +6

Impacted Files	Coverage Δ
src/Utilities/copy.jl	94.66% <0%> (ø)	⬆️

---

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 9b259fa...b8b3cd3. Read the comment docs.