Fixed broken link in README.md

[ajchili]

This PR addresses and fixes issue #1479. The link to documentation appears to have changed and when clicking the Pipelines API doc link within the README file a 404 error is presented. This change fixes the link within the README file.

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This change is 

[k8s-ci-robot]

Hi @ajchili. Thanks for your PR.

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[gaoning777]

/ok-to-test

[gaoning777]

/lgtm

[IronPan]

/lgtm
/approve

[k8s-ci-robot]

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