Clarify unary for Sass (#17595)

* Fix: Clarify SCSS

* Try other interpretation

It seems my initial fix _did_ break the styling. This commit tries the other
interpretation to see if it passes the snapshot tests.

* Revert "Try other interpretation"

This reverts commit 8e1ac73.

---------

Co-authored-by: Taylor Jones <tay1orjones@users.noreply.github.com>

packages/styles/scss/components/modal/_modal.scss

@@ -125,7 +125,7 @@
 
   // fluid form in modal
   .#{$prefix}--modal-content .#{$prefix}--form--fluid {
-    margin-inline: -$spacing-05 -$spacing-05;
+    margin-inline: -$spacing-05 - $spacing-05;
   }
 
   //TO-DO: remove .#{$prefix}--modal-content__regular-content in v11 since hasForm has been deprecated