Update signatures in readme to use AbstractVector

[m-fila]

The signatures in readme still mention Vector{T} instead of AbstractVector{T}
This is follow up to #150 - we forgot to update readme 😁

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.07%. Comparing base (0f47837) to head (8089065).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #154   +/-   ##
=======================================
  Coverage   75.07%   75.07%           
=======================================
  Files          19       19           
  Lines        1252     1252           
=======================================
  Hits          940      940           
  Misses        312      312           

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