Add namespace for httpbin-gateway in istio doc

[yangy2000]

Signed-off-by: 杨阳 10014842 yangyang1@zte.com.cn

Description

Fixes #ISSUE

Checklist

• Unit tests updated
• End user documentation updated

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[Raffo]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Raffo, yangy2000

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[seanmalloy]

/kind documentation