Update and rename Readme.md to README.md

Fixing case-sensitive link from main Readme for Pixeltable README.md.

Author: pierrebrunelle

third_party/Pixeltable/Readme.md third_party/Pixeltable/README.md

@@ -16,4 +16,4 @@ A [Hugging Face Space](https://huggingface.co/spaces/Pixeltable/Prompt-Engineeri
 pip install pixeltable mistralai
 ```
 
-See how to work with [Mistral AI in Pixeltable](https://docs.pixeltable.com/docs/mistral-ai)
+See how to work with [Mistral AI in Pixeltable](https://docs.pixeltable.com/docs/mistral-ai)