[Docs] Add example for building Personal AI Assistant using Mem0 (#1486)

docs/examples/personal-ai-tutor.mdx

@@ -23,6 +23,9 @@ Below is the complete code to create and interact with a Personalized AI Tutor u
 from openai import OpenAI
 from mem0 import Memory
 
+# Set the OpenAI API key
+os.environ['OPENAI_API_KEY'] = 'sk-xxx'
+
 # Initialize the OpenAI client
 client = OpenAI()