fix a python typo

Jenkinsfile

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