@bcworkz is correct. To elaborate, you will want to add a bit of CSS code. You can do this by going to the Customize view (link is in the WordPress sidebar). You will then see a menu item on the left called Additional CSS. Open that up to reveal a little text editor to add some odds and ends CSS when needed.
I believe the following sets of CSS should do the trick with a few caveats.
The following code will resolve the issue across your entire site for that particular block type. I don’t really see a downside to this, but if you see something break on another page, you might try one of the other examples I am providing.
@media (max-width: 600px) {
.wp-block-media-text {
display: block;
}
}
The following two sets are a little more focused in their target. This next bit of code targets that particular block, but only on blog posts.
@media (max-width: 600px) {
.single-post .wp-block-media-text {
display: block;
}
}
This last one only targets the specific page you linked.
@media (max-width: 600px) {
.postid-9267 .wp-block-media-text {
display: block;
}
}
