Add note about "unbalanced" parentheses in scanning stage

Author: rohitpaulk

course-definition.yml

@@ -254,6 +254,7 @@ stages:
       ### Notes
 
       - This output format matches the spec in the [book's repository](https://github.com/munificent/craftinginterpreters/tree/01e6f5b8f3e5dfa65674c2f9cf4700d73ab41cf8/test/scanning)
+      - When scanning for tokens, it's valid to have "unbalanced" parentheses. When we get to parsing expressions in later stages, these cases will be highlighted as errors.
 
     # A description of this stage that is used on the course overview page and other marketing material. Markdown supported.
     marketing_md: |-