doc: correct field name in example config

[iwanaga]

Corrected an error in the config example.

Confirmed that the validation now passes as shown below.
Invalid when we use from_cookies as a field name:

k apply --dry-run=server -f test-jwt.yaml
Error from server (BadRequest): error when creating "test-jwt.yaml": RequestAuthentication in version "v1" cannot be handled as a RequestAuthentication: strict decoding error: unknown field "spec.jwtRules[0].from_cookies"

Valid when we use fromCookies:

k apply --dry-run=server -f test-jwt.yaml
requestauthentication.security.istio.io/jwt-on-ingress created (server dry run)

[istio-policy-bot]

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[istio-testing]

Hi @iwanaga. Thanks for your PR.

I'm waiting for a istio member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[ilrudie]

/ok-to-test

[ilrudie]

/test release-notes

[ilrudie]

LGTM, Thank you!