Fix python version in serverless image resizer quickstart

[jcollado]

Serverless image resizer quickstar provides the following tip:

If you are encountering issues with the installation of the packages, such as Pillow, ensure you use the same version as the Python Lambdas (3.9) for Pillow to work.

However, when I tried to follow that advice, the resize lambda failed for me. When I looked at the sample-serverless-image-resizer-s3-lambda repository, I realized that current code uses python 3.11 instead. After updating my environment to use python 3.11, the code worked as expected.

To fix the problem for future readers of the documentation, the references to python 3.9 should be updated to python 3.11.