correct liebility typo in overview.md

[MattHJensen]

Closes #2454.

[codecov]

Codecov Report

Merging #2457 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2457   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files          13       13           
  Lines        2582     2582           
=======================================
  Hits         2580     2580           
  Misses          2        2           

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