Update ambassador doc for shadow deployment

[anggao]

What this PR does / why we need it:
seldon.io/ambassador-shadow:true is obsoleted

Which issue(s) this PR fixes:

Fixes #1741

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[seldondev]

Hi @anggao. Thanks for your PR.

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[anggao]

/cc @cliveseldon

[ukclivecox]

/approve

[seldondev]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cliveseldon

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