Update layer.md

Co-authored-by: Brandon Mitchell <git@bmitch.net>
Signed-off-by: Andrew Consroe <aconz2@gmail.com>

layer.md

@@ -258,7 +258,7 @@ b/
 c/file3
 ```
 
-If we then delete `file1`, `file2`, and `b/`, the next layer looks like:
+If we then delete `file1`, `file2`, and `b/`, while leaving `file3` and adding `file4, the next layer looks like:
 
 ```text
 .wh.file1