fix: correct formats where conversion is allowed

[hbagdi]

Previously, KongGateway was listed in --to and Konnect was listed in
--to, which wasn't supported.
This fixes the documentation so that the user isn't confused anymore.

[codecov-commenter]

Codecov Report

Merging #460 (bc00b6d) into main (5ce9ef2) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #460      +/-   ##
==========================================
+ Coverage   50.28%   50.29%   +0.01%     
==========================================
  Files          72       72              
  Lines        5698     5700       +2     
==========================================
+ Hits         2865     2867       +2     
  Misses       2501     2501              
  Partials      332      332              

Impacted Files	Coverage Δ
cmd/convert.go	42.30% <100.00%> (+4.80%)	⬆️

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[rainest]

IMO including the []convert.Format{...} inline with the Sprint would be more readable if we don't need to reuse the format lists, but that's a minor style nit that doesn't really matter--the variable definitions are close by, so vov.