default-repo config documentation

[balopat]

Fixing the nits in #1057.

[codecov-io]

Codecov Report

Merging #1194 into master will decrease coverage by 0.02%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #1194      +/-   ##
==========================================
- Coverage   41.47%   41.45%   -0.03%     
==========================================
  Files          95       95              
  Lines        4229     4229              
==========================================
- Hits         1754     1753       -1     
- Misses       2304     2305       +1     
  Partials      171      171

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/config/util.go	33.73% <ø> (-0.79%)	⬇️
pkg/skaffold/util/image.go	100% <ø> (ø)	⬆️
cmd/skaffold/app/cmd/config/list.go	0% <0%> (ø)	⬆️
pkg/skaffold/deploy/helm.go	65.56% <100%> (ø)	⬆️

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