chore: updates assignment instructions for lesson_06 (#300)

Signed-off-by: Anthony D. Mays <[email protected]>

Author: anthonydmays

lesson_06/README.md

@@ -36,7 +36,7 @@ npm start
 ```bash
 npm run check
 ```
-1. You must only submit changes to the `expression_calculator.ts` file to receive full credit.
+1. You must only submit changes to the `expression_calculator.ts` and `.env.test` files to receive full credit.
 
 ### Stretch Assignment