fix `parse(Decimal, "1e0")`

[oscardssmith]

Off by 1 in the branch for the exponent.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (01a4884) 96.29% compared to head (19ff2a3) 99.21%. Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
+ Coverage   96.29%   99.21%   +2.92%     
==========================================
  Files           6        6              
  Lines         108      128      +20     
==========================================
+ Hits          104      127      +23     
+ Misses          4        1       -3     

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