Fix RangeEdit step min

[tlambert03]

fixes #699

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.73%. Comparing base (e48fc4b) to head (1ff608e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/magicgui/widgets/_concrete.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #701      +/-   ##
==========================================
- Coverage   88.74%   88.73%   -0.01%     
==========================================
  Files          40       40              
  Lines        4807     4812       +5     
==========================================
+ Hits         4266     4270       +4     
- Misses        541      542       +1     

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