Simple Task

If it's not very important, why would it not go on the Product Backlog? Context switching is very expensive. Only a Developer would be able to confirm that it is indeed a small modification, so you'd be asking a Developer to context switch away from important and valuable work to look at and understand the modification, confirm that it's small, and then do that unimportant thing.

Considering that a Product Owner's responsibility is to maximize the value of the work done by the Scrum Team, I can't see how asking for the modification would be consistent with that responsibility.

if the PO asks for a small and not very important modification that won't even take one minute we can do it without putting it in the PB and it can be realized at any sprint!!

What is the effect of this practice on transparency?

One minute of coding is only part of the time it takes to do anything. That developer has to switch stash the work that they are doing now, create a new branch, make a code change, test the code change, submit the code change to the source code repository, wait for the build process to complete, deploy the change to production, etc.  Then the developer will have to context switch back to the original work that they were doing. 

This can be disruptive and should be VERY CAREFULLY weighed against the work that is already in progress.  Something simple usually isn't something that is critical to have done so it can usually wait until at least the next sprint.