Fix mutation rate in time units example

[lkirk]

The branch diversity stat was being scaled by 1e-8 instead of 2e-8, leading to the underestimation of branch diversity when scaled by generation time.

I've corrected the code example and the text.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.65%. Comparing base (4cb44e2) to head (9d53350).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #344   +/-   ##
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  Coverage   95.65%   95.65%           
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  Files           8        8           
  Lines         621      621           
  Branches      131      131           
=======================================
  Hits          594      594           
  Misses         18       18           
  Partials        9        9           

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[petrelharp]

Good catch - and thanks for the PR! Here's where mutation rate is indeed set to 2e-8.
(The test failure is a transient CI failure; I'll merge this.)