fix(tuto): fix issue with protocol specification

Author: bene2k1

tutorials/installation-uncomplicated-firewall/index.mdx

@@ -9,7 +9,7 @@ tags: Firewall UFW port-25
 categories:
   - instances
 dates:
-  validation: 2024-12-13
+  validation: 2025-01-30
   posted: 2018-07-18
 ---
 
@@ -129,9 +129,13 @@ As the firewall is running now, it is possible to add more rules to it:
 Allow the connection to port 25 (SMTP) via TCP to the server:
 
 ```code
-sudo ufw allow 25/TCP
+sudo ufw allow 25/tcp
 ```
 
+<Message type="tip">
+  The protocol specification (`tcp`) is case-sensitive. Make sure to use only lowercase letters when specifying the protocol.
+</Message>
+
 ## Deleting rules
 
 Over time, you may recognize that some of the rules you defined previously do not match your requirements anymore.