Use more recent nginx in example

Signed-off-by: Stefan Schindler <dns2utf8@estada.ch>

examples/nginx-app/base.yaml

@@ -37,7 +37,7 @@ spec:
         app: nginx
     spec:
       containers:
-      - image: nginx:1.7.9
+      - image: nginx:1.17.6
         name: nginx
         ports:
         - containerPort: 80

examples/nginx-app/with-pv.yaml

@@ -29,6 +29,7 @@ metadata:
   labels:
     app: nginx
 spec:
+  # Optional:
   # storageClassName: <YOUR_STORAGE_CLASS_NAME>
   accessModes:
     - ReadWriteOnce
@@ -62,7 +63,7 @@ spec:
           persistentVolumeClaim:
            claimName: nginx-logs
       containers:
-      - image: nginx:1.7.9
+      - image: nginx:1.17.6
         name: nginx
         ports:
         - containerPort: 80