Fix typo and emphasize for clarity

Fixes #291 (comment)

spec/2023-07-draft.md

@@ -619,8 +619,8 @@ In this case it is recommended to symlink identical files from these subdirector
 The `data/sample` directory contains test cases similar to those in `data/secret`.
 Every submission is run against these.
 
-`data/sample` must not contain test groups. 
-Also, it does not contain any test cases (for example when the samples shown in the PDF are not actually valid cases).
+`data/sample` must not contain test groups.
+Also, it does not _have to_ contain any test cases, for example when the samples shown in the PDF are not actually valid cases.
 
 #### Samples shown in the problem statement