Minimal help string changes for --repository[-url]

[jamadden]

Fixes #265.

I didn't touch the AUTHORS file because this is such a trivial change.

[codecov]

Codecov Report

Merging #266 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #266   +/-   ##
=======================================
  Coverage   67.36%   67.36%           
=======================================
  Files          12       12           
  Lines         573      573           
  Branches       91       91           
=======================================
  Hits          386      386           
  Misses        161      161           
  Partials       26       26

Impacted Files	Coverage Δ
twine/commands/upload.py	61.9% <ø> (ø)	⬆️
twine/utils.py	84.53% <ø> (ø)	⬆️
twine/commands/register.py	0% <ø> (ø)	⬆️

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[sigmavirus24]

Thanks @jamadden!