Make usage messages look like the others.

[dgageot]

Signed-off-by: David Gageot david@gageot.net

[codecov-io]

Codecov Report

Merging #1267 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #1267   +/-   ##
=======================================
  Coverage   42.42%   42.42%           
=======================================
  Files          96       96           
  Lines        4247     4247           
=======================================
  Hits         1802     1802           
  Misses       2269     2269           
  Partials      176      176

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/dev.go	0% <0%> (ø)	⬆️
cmd/skaffold/app/cmd/config/flags.go	0% <0%> (ø)	⬆️

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