docs(courses): add missing environment variable name (#138)

[xodn348]

Summary

Fixes #138

The setup instructions in the API fundamentals course tell users to create a .env file but don't explicitly mention the required variable name (ANTHROPIC_API_KEY) until a code block further down. This can confuse users who skim the instructions.

Changes

• Updated the .env file creation instruction in anthropic_api_fundamentals/01_getting_started.ipynb to explicitly mention ANTHROPIC_API_KEY inline at the point where users are first told to create the file.

Before:

Create a new file called '.env' in the same directory as your notebook.

After:

Create a new file called '.env' in the same directory as your notebook. This file will hold your 'ANTHROPIC_API_KEY' environment variable.