fix a python typo

Jenkinsfile

@@ -1,21 +1,15 @@
-pipeline {
+node {
-    agent none
+    /* Requires the Docker Pipeline plugin to be installed */
-    stages {
+
     stage('Back-end') {
-            agent {
+        docker.image('python:3-alpine').inside {
-                docker { image 'python:3-alpine' }
-            }
-            steps {
             sh 'python --version'
         }
     }
+
     stage('Front-end') {
-            agent {
+        docker.image('node:14-alpine').inside {
-                docker { image 'node:14-alpine' }
-            }
-            steps {
             sh 'node --version'
         }
     }
-    }
 }