[Fix] display-name: fix identifying _ as a capital letter

[apbarrero]

Fix #3334

[codecov]

Codecov Report

Merging #3335 (6b42731) into master (87fb344) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3335   +/-   ##
=======================================
  Coverage   97.71%   97.71%           
=======================================
  Files         123      123           
  Lines        8788     8788           
  Branches     3190     3190           
=======================================
  Hits         8587     8587           
  Misses        201      201           

Impacted Files	Coverage Δ
lib/util/isFirstLetterCapitalized.js	100.00% <100.00%> (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 87fb344...6b42731. Read the comment docs.

[ljharb]

Thanks!