Write out 'curl -X' in Jenkinsfile (#7081)

Write out -X in cURL

Signed-off-by: Alexander Brandes <mc.cache@web.de>

Jenkinsfile

@@ -113,8 +113,8 @@ node('docker&&linux') {
         }
         stage('Purge cached CSS') {
             sh '''
-            curl -X PURGE https://www.jenkins.io/css/jenkins.css
-            curl -X PURGE https://www.jenkins.io/stylesheets/styles.css
+            curl --request PURGE https://www.jenkins.io/css/jenkins.css
+            curl --request PURGE https://www.jenkins.io/stylesheets/styles.css
             '''
         }
     }