Update custom-resource-definitions.md

The example on line 746-754, should has an image tag; with the image tag, the validation will fail with the error "validating data: ValidationError(CronTab.spec): unknown field "image"".  Solution : Adding the image field on the CustomResourceDefinitions.

content/en/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions.md

@@ -673,6 +673,8 @@ spec:
                 cronSpec:
                   type: string
                   pattern: '^(\d+|\*)(/\d+)?(\s+(\d+|\*)(/\d+)?){4}$'
+                 image:
+                  type: string
                 replicas:
                   type: integer
                   minimum: 1
@@ -720,6 +722,8 @@ spec:
             cronSpec:
               type: string
               pattern: '^(\d+|\*)(/\d+)?(\s+(\d+|\*)(/\d+)?){4}$'
+            image:
+              type: string
             replicas:
               type: integer
               minimum: 1