Python: Fix typo in line to instantiate Kernel. (#6020)

Kernel is already imported and there's no sk reference anymore since
it was removed last week in:

Python: fixing python readme
(https://github.com/microsoft/semantic-kernel/pull/5903)

### Motivation and Context

<!-- Thank you for your contribution to the semantic-kernel repo!
Please help reviewers and future users, providing the following
information:
  1. Why is this change required?
The change https://github.com/microsoft/semantic-kernel/pull/5903
imported Kernel directly so the code
no longer works as written

  2. What problem does it solve?
Code sample does not run
  3. What scenario does it contribute to?
sample code in README.md working
  4. If it fixes an open issue, please link to the issue here.
 None
-->

### Description

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### 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 😄

python/README.md

@@ -39,7 +39,7 @@ from semantic_kernel.connectors.ai.open_ai import OpenAIChatCompletion, AzureCha
 from semantic_kernel.prompt_template import PromptTemplateConfig
 from semantic_kernel.utils.settings import openai_settings_from_dot_env, azure_openai_settings_from_dot_env
 
-kernel = sk.Kernel()
+kernel = Kernel()
 
 # Prepare OpenAI service using credentials stored in the `.env` file
 api_key, org_id = openai_settings_from_dot_env()