top: fix warning from manual_saturating_arithmetic

[cakebaker]

This PR fixes a warning from the manual_saturating_arithmetic lint

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 18.78%. Comparing base (6ac0e01) to head (60a0f1d).
⚠️ Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
src/uu/top/src/tui/mod.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #647      +/-   ##
==========================================
+ Coverage   18.77%   18.78%   +0.01%     
==========================================
  Files          66       66              
  Lines        9752     9745       -7     
  Branches      516      517       +1     
==========================================
  Hits         1831     1831              
+ Misses       7921     7914       -7     

Flag	Coverage Δ
macos_latest	6.91% <0.00%> (ø)
ubuntu_latest	19.15% <0.00%> (+0.01%)	⬆️
windows_latest	0.02% <0.00%> (+<0.01%)	⬆️

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[cakebaker]

I merged this PR myself to get the CI back to green.