hugetop: optimize time format display

[Franklin-Qi]

Change the timestamp to a time format that is readable by humans.

Closed: #685

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.21%. Comparing base (c1801fd) to head (a995b90).
⚠️ Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
src/uu/hugetop/src/hugetop.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #686   +/-   ##
=======================================
  Coverage   19.20%   19.21%           
=======================================
  Files          68       68           
  Lines       10155    10150    -5     
  Branches      546      546           
=======================================
  Hits         1950     1950           
+ Misses       8205     8200    -5     

Flag	Coverage Δ
macos_latest	6.74% <0.00%> (+<0.01%)	⬆️
ubuntu_latest	19.56% <0.00%> (+<0.01%)	⬆️
windows_latest	0.02% <0.00%> (+<0.01%)	⬆️

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