fix typo, add stringdist-metrics link

[ChrisMuir]

If you'd prefer to use a different url for the stringdist-metrics link, let me know, I'd be happy to edit.

[codecov-io]

Codecov Report

Merging #40 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #40   +/-   ##
=======================================
  Coverage   91.11%   91.11%           
=======================================
  Files           9        9           
  Lines         394      394           
=======================================
  Hits          359      359           
  Misses         35       35

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[dgrtwo]

Thanks for this!