Fix argument type of fit_enc_sigmas

[fhagemann]

I tried to test this fit_enc_sigmas by passing the second argument as (1.0:1.0:20.0)u"µs", but it failed.. this little change in the type of the argument type seems to fix it.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 28.39%. Comparing base (391ed7e) to head (4a43dfa).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/filter_optimization.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #128   +/-   ##
=======================================
  Coverage   28.39%   28.39%           
=======================================
  Files          37       37           
  Lines        3564     3564           
=======================================
  Hits         1012     1012           
  Misses       2552     2552           

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[theHenks]

Thanks!