Fix Lisi return type and docstring

[adamgayoso]

Closes #113

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.78%. Comparing base (e31aa1c) to head (cf5585c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #182   +/-   ##
=======================================
  Coverage   90.78%   90.78%           
=======================================
  Files          26       26           
  Lines         977      977           
=======================================
  Hits          887      887           
  Misses         90       90           

Files with missing lines	Coverage Δ
src/scib_metrics/metrics/_lisi.py	100.00% <100.00%> (ø)