Fix running single analyzer which isn't a rule bug

[ldemailly]

Fixes #1230
(also a few typo and unclear docs while tracking down the code)

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 68.53%. Comparing base (a836898) to head (55883aa).

Files with missing lines	Patch %	Lines
cmd/gosec/main.go	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1231   +/-   ##
=======================================
  Coverage   68.53%   68.53%           
=======================================
  Files          75       75           
  Lines        4382     4382           
=======================================
  Hits         3003     3003           
  Misses       1232     1232           
  Partials      147      147           

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