[Python Getting Started] Update base_url to endpoint (#5765)

### Motivation and Context

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1. This change helps users by being able to copy the Azure OpenAI
endpoint directly
2. This solves the problem of getting a 404 Resource Not Found when
using `base_url`
3. Getting Started in Python scenario

### Description

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change `base_url` to `endpoint`

### Contribution Checklist

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- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

Co-authored-by: Eduard van Valkenburg <[email protected]>

python/README.md

@@ -57,7 +57,7 @@ kernel.add_service(
 #   AzureChatCompletion(
 #       service_id="dv",
 #       deployment_name=deployment,
-#       base_url=endpoint,
+#       endpoint=endpoint,
 #       api_key=api_key
 #   )
 # )