Revert change to spacing class names.

assets/sass/_sass_variables.scss

@@ -51,8 +51,8 @@ $breakpoints: (
 );
 
 // Spacing
-// Generates mt-m (mt = margin-top), mb-m (mb = margin-bottom) and
-// mtb-m (mtb = margin-block) classes.
+// Generates mt--m (mt = margin-top), mb--m (mb = margin-bottom) and
+// mx--m (mx = margin-block) classes.
 $spacing: (
   xxl:        3,
   xl:         2,

assets/sass/components/spacing/_spacing.scss

@@ -1,13 +1,13 @@
 @each $name, $space in $spacing {
-  .mt-#{$name} {
+  .mt--#{$name} {
     @include margin-block-start($space);
   }
 
-  .mb-#{$name} {
+  .mb--#{$name} {
     @include margin-block-end($space);
   }
 
-  .mx-#{$name} {
+  .mx--#{$name} {
     @include margin-block($space);
   }
 }