docs(lambda): Use bundlingImage rather than bundlingDockerImage (#13945)

Resolves #13944
----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-lambda/README.md

@@ -451,19 +451,19 @@ Example with Python:
 new lambda.Function(this, 'Function', {
   code: lambda.Code.fromAsset(path.join(__dirname, 'my-python-handler'), {
     bundling: {
-      image: lambda.Runtime.PYTHON_3_6.bundlingDockerImage,
+      image: lambda.Runtime.PYTHON_3_8.bundlingImage,
       command: [
         'bash', '-c',
         'pip install -r requirements.txt -t /asset-output && cp -au . /asset-output'
       ],
     },
   }),
-  runtime: lambda.Runtime.PYTHON_3_6,
+  runtime: lambda.Runtime.PYTHON_3_8,
   handler: 'index.handler',
 });
 ```
 
-Runtimes expose a `bundlingDockerImage` property that points to the [AWS SAM](https://github.com/awslabs/aws-sam-cli) build image.
+Runtimes expose a `bundlingImage` property that points to the [AWS SAM](https://github.com/awslabs/aws-sam-cli) build image.
 
 Use `cdk.DockerImage.fromRegistry(image)` to use an existing image or
 `cdk.DockerImage.fromBuild(path)` to build a specific image:

packages/@aws-cdk/aws-lambda/test/integ.bundling.ts

@@ -18,7 +18,7 @@ class TestStack extends Stack {
     const fn = new lambda.Function(this, 'Function', {
       code: lambda.Code.fromAsset(assetPath, {
         bundling: {
-          image: lambda.Runtime.PYTHON_3_6.bundlingDockerImage,
+          image: lambda.Runtime.PYTHON_3_6.bundlingImage,
           command: [
             'bash', '-c', [
               'cp -au . /asset-output',