Fix small typo in using_docker_images.md, file is called `build_script` (using underscore)

## What does this MR do?

This MR fixes a very small typo in the using_docker_images.md documentation, a space was used in the filename `build_script` while it should be an underscore. Not sure of a MR is the best way to get this changed, trying it anyway :).

## Moving docs to a new location?

No

See merge request !6682