Isn't that "Round off"? i couldn't find the details. I remember I had read it somewhere before.

If it says "Round down", then you are right. The chance would be 1 in 4,294,967,295, as only 1 decimal number can be rounded to 1 billion.
If it says "Round off", then there are other decimal numbers that can be rounded to 1 billion. In this case, the chance would be 3 in 4,294,967,295 which is equal to 1 in 1,431,655,765.

Whether it's 1 in 4,294,967,295 or 3 in 4,294,967,295, my previous calculation (0.5 in 1,000,000,000) was wrong.