Update error message to use are since manifests is plural

[pdbrito]

Description
A small fix to update the grammar of the error message returned when no kubernetes manifest files can be found.

Before:
FATA[0000] one or more Kubernetes manifests is required to run skaffold

After:
FATA[0000] one or more Kubernetes manifests are required to run skaffold

[codecov]

Codecov Report

Merging #3930 into master will increase coverage by 0.08%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
pkg/skaffold/initializer/deploy/init.go	68.96% <ø> (ø)
pkg/skaffold/deploy/resource/deployment.go	90.90% <0.00%> (-3.69%)	⬇️
pkg/skaffold/build/jib/jib.go	72.99% <0.00%> (-1.28%)	⬇️
pkg/skaffold/docker/dependencies.go	71.83% <0.00%> (-1.09%)	⬇️
pkg/skaffold/build/list/list.go	70.58% <0.00%> (-0.85%)	⬇️
pkg/skaffold/deploy/status_check.go	68.30% <0.00%> (-0.76%)	⬇️
pkg/skaffold/util/util.go	85.50% <0.00%> (-0.51%)	⬇️
pkg/skaffold/debug/transform_python.go	84.70% <0.00%> (-0.36%)	⬇️
cmd/skaffold/app/cmd/init.go	100.00% <0.00%> (ø)
cmd/skaffold/app/cmd/find_configs.go	45.71% <0.00%> (ø)
... and 11 more

[dgageot]

LGTM