change the directory of default-backend.yaml

[davidstack]

there is no default-backend.yaml in examples directory,so we should change the doc.

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[k8s-reviewable]

This change is 

[davidstack]

i have signed CNCF Individual Contributor License Agreement

[coveralls]

Coverage increased (+0.03%) to 46.939% when pulling 7321704 on davidstack:master into 8ca5fbe on kubernetes:master.

[aledbf]

/lgtm

[aledbf]

@davidstack thanks!