Fix typo in traversal docs

[juliohm]

I believe this is a typo, but didn't check the source code. It doesn't make sense to have a default cost of 1.0 when the user assigns the value 0.0.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.16%. Comparing base (9a7a6df) to head (9305f0c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #469   +/-   ##
=======================================
  Coverage   97.16%   97.16%           
=======================================
  Files         121      121           
  Lines        7020     7020           
=======================================
  Hits         6821     6821           
  Misses        199      199           

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