Fix n-down media query mixin generator

[good-idea]

Hiya,

I accidentally pushed my intended change to main. see here

Then I was an impatient bad boy and published at 1.2.1

Using a mixin like @include media('md-down') { ... } wouldn't work because the SASS variable inside calc(...) needs to be escaped.

I've used yarn link to test this locally and it's now doing the trick.

Assuming all is well, we just need an updated changelog. I was going to add this manually but it looks like it's generated somehow? So I didn't want to fuss.