How to sraigthen the end of a line

Hi everyone,

I'm pretty new to Illustrator and I need a little help with something.

I got a design for a logo, so the background needs to be transparent.

In my first design ideas I used a white circle, put a black box above it and then just wrote over it.
As you can see the end of the white line from the circle is straight.

But when I combine the box and the circle, so that I can have a transparent background later, it turns out like this:

I want the end of the white line to be straight. How do I do that? Can I do that?