docs: update system template instructions

- remove duplicate "Signed off" removal instruction
- clarify instruction numbering for better readability

Signed-off-by: mingcheng <[email protected]>

Author: mingcheng

templates/system.txt

@@ -63,7 +63,7 @@ Please generate a git commit message with the specified requirements.
 
 9. Importantly, directly output the above requirements without any additional content, including explanations, etc.
 
-8. If the "Signed off" message or flag is present, remove it.
+10. Importantly, If the "Signed off" message or flag is present, remove it.
 
 Finally, Here are a few examples that will be demonstrated below.