Try fix website pr playground (#4076)

eng/tsp-core/pipelines/pr-tools.yml

@@ -27,7 +27,7 @@ extends:
           - job: tryit
             displayName: Setup Try It
             variables:
-              TYPESPEC_WEBSITE_BASE_PATH: "prs/$(System.PullRequest.PullRequestNumber)/"
+              TYPESPEC_WEBSITE_BASE_PATH: "/prs/$(System.PullRequest.PullRequestNumber)/"
             steps:
               - checkout: self
                 persistCredentials: true