fix misplaced bracket for maps

[flyingapfopenguin]

The is_surjective function for NumFiledHom has a misplaced bracket.

[codecov]

Codecov Report

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

Project coverage is 75.90%. Comparing base (8532b39) to head (cf0ef62).
Report is 3 commits behind head on master.

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

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1686       +/-   ##
===========================================
+ Coverage    6.39%   75.90%   +69.51%     
===========================================
  Files         362      362               
  Lines      114109   114305      +196     
===========================================
+ Hits         7293    86759    +79466     
+ Misses     106816    27546    -79270     

Files with missing lines	Coverage Δ
src/Map/NumberField.jl	72.53% <0.00%> (+71.73%)	⬆️

... and 323 files with indirect coverage changes

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