Update app.py

Updated COHERE_API_KEY to CO_API_KEY.

app.py

@@ -17,7 +17,7 @@ description = "Learn a New Language With Aya"
 
 # Load environment variables
 load_dotenv()
-COHERE_API_KEY = os.getenv('COHERE_API_KEY')
+COHERE_API_KEY = os.getenv('CO_API_KEY')
 SEAMLESSM4T = os.getenv('SEAMLESSM4T')
 
 co = cohere.Client(COHERE_API_KEY)