fix: fix URL to PR, for PR labeling automation

[rahalan]

On PR created trigger, GH provides the URL to the PR through the 'github.event.pull_request.url' variable. Unfortunately, there is a spelling issue in it: it has an "s" too much in it (pulls instead of pull), see 'https://github.com/Azure/bicep-registry-modules/pull**s**/3016'. Because of that, the PR labeling logic fail, a it can't find the PR

[microsoft-github-policy-service]

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[microsoft-github-policy-service]

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