Fix FromAsCasing warning

Change FROM python:${PYTHON_VERSION}-slim as base line in Dockerfile content to get rid of warning:
 => WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 3)

Which shows up on image build.

content/language/python/containerize.md

@@ -77,7 +77,7 @@ Create a file named `Dockerfile` with the following contents.
 # Want to help us make this template better? Share your feedback here: https://forms.gle/ybq9Krt8jtBL3iCk7
 
 ARG PYTHON_VERSION=3.11.4
-FROM python:${PYTHON_VERSION}-slim as base
+FROM python:${PYTHON_VERSION}-slim AS base
 
 # Prevents Python from writing pyc files.
 ENV PYTHONDONTWRITEBYTECODE=1