Fix wrong library attribution in the comment for Append To List (#96)

Append To List is from Collections Library : not BuiltIn Library 


https://robotframework.org/robotframework/latest/libraries/Collections.html#Append%20To%20List

The inline comment in chapter-05/02_control_structures.md:297 says
Append To List is a BuiltIn keyword, but it actually belongs to the
[Collections](https://robotframework.org/robotframework/latest/libraries/Collections.html#Append%20To%20List)
library

(already imported on line 267 of the same example) ,, for a learner,
could create confusion about which library each keyword comes from.


if the intent was to say it is a built-in keyword ( a keyword that come
with RF without installing any other dependency ) and we want to keep
this intent, i have two suggestions :
- Standard library keyword to append a value to a list (not very clear)
-   built-in ( written this way ) keyword to append a value to a list

Co-authored-by: Gerwin <8427040+GerwinLaagland@users.noreply.github.com>

Author: hassineabd

website/docs/chapter-05/02_control_structures.md

@@ -294,7 +294,7 @@ Get Older Participants
         Log    Participant ${participant.name} is older than 40
         # ^ Logs participant name if age is above the minimum
         Append To List    ${older_participants}    ${participant}
-        # ^ BuiltIn keyword to append a value to a list
+        # ^ Collections keyword to append a value to a list
     END
     RETURN    ${older_participants}
 ```