Docs - Update capture group placeholder

[kppullin]

What this PR does / why we need it:

In the rewrite doc, the current ingress example uses the $2 capture group placeholder, however the description refers to the $1 placeholder (this was previously correct, but was not updated when the ingress example changed from $1 to $2).

[k8s-ci-robot]

Hi @kppullin. Thanks for your PR.

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[aledbf]

/ok-to-test

[aledbf]

/lgtm
/approve

[aledbf]

@kppullin thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, kppullin

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