Fix generic warning at Xcode 15

[humdrum]

What this PR does / why we need it:

• Fix misused generic declaration.
  • It has no problem now. but could be an error with Swift 6

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (f15db57) 43.11% compared to head (21a9f4f) 43.11%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #122   +/-   ##
=======================================
  Coverage   43.11%   43.11%           
=======================================
  Files         103      103           
  Lines       22813    22813           
=======================================
  Hits         9835     9835           
  Misses      12978    12978           

Files	Coverage Δ
Sources/Util/LLRBTree.swift	86.68% <89.47%> (ø)

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[myupage]

👍