Update LLM_Prompt_Injection_Prevention_Cheat_Sheet.md

[KadirArslan]

I've added RAG poisoning attack to the LLM Prompt Injection CS.

Thank you for submitting a Pull Request (PR) to the Cheat Sheet Series.

🚩 If your PR is related to grammar/typo mistakes, please double-check the file for other mistakes in order to fix all the issues in the current cheat sheet.

Please make sure that for your contribution:

• In case of a new Cheat Sheet, you have used the Cheat Sheet template.
• All the markdown files do not raise any validation policy violation, see the policy.
• All the markdown files follow these format rules.
• All your assets are stored in the assets folder.
• All the images used are in the PNG format.
• Any references to websites have been formatted as [TEXT](URL)
• You verified/tested the effectiveness of your contribution (e.g., the defensive code proposed is really an effective remediation? Please verify it works!).
• The CI build of your PR pass, see the build status here.

If your PR is related to an issue, please finish your PR text with the following line:

This PR fixes issue #<REPLACE WITH ISSUE NUMBER>.

AI Tool Usage Disclosure (required for all PRs)

Please select one of the following options:

• I have NOT used any AI tool to generate the contents of this PR.
• I have used AI tools to generate the contents of this PR. I have verified
  the contents and I affirm the results. The LLM used is [llm name and version]
  and the prompt used is [your prompt here]. [Feel free to add more details if needed]

[mackowski]

Thanks! Good addition @KadirArslan!

[mackowski]

@maheshkukreja do you want to review? It is update to CS that if I remember correctly you authored