Correct the syntax in hero is-bold colour variants

While the prior syntax does produce valid css, it's not quite valid scss
and so produces warnings in various pre and post-processing pipelines
which various upstream consumers of Bulma may use.

Author: bagedevimo

sass/layout/hero.scss

@@ -122,19 +122,19 @@ $hero-colors: dv.$colors !default;
       // Modifiers
       &.#{iv.$class-prefix}is-bold {
         $gradient-top-left: hsl(
-          calc(#{cv.getVar("hero-h")} - $hero-gradient-h-offset),
-          calc(#{cv.getVar("hero-s")} + $hero-gradient-s-offset),
-          calc(#{cv.getVar("hero-background-l")} + $hero-gradient-l-offset)
+          calc(#{cv.getVar("hero-h")} - #{$hero-gradient-h-offset}),
+          calc(#{cv.getVar("hero-s")} + #{$hero-gradient-s-offset}),
+          calc(#{cv.getVar("hero-background-l")} + #{$hero-gradient-l-offset})
         );
         $gradient-middle: hsl(
           #{cv.getVar("hero-h")},
           #{cv.getVar("hero-s")},
           #{cv.getVar("hero-background-l")}
         );
         $gradient-bottom-right: hsl(
-          calc(#{cv.getVar("hero-h")} + $hero-gradient-h-offset),
-          calc(#{cv.getVar("hero-s")} - $hero-gradient-s-offset),
-          calc(#{cv.getVar("hero-background-l")} - $hero-gradient-l-offset)
+          calc(#{cv.getVar("hero-h")} + #{$hero-gradient-h-offset}),
+          calc(#{cv.getVar("hero-s")} - #{$hero-gradient-s-offset}),
+          calc(#{cv.getVar("hero-background-l")} - #{$hero-gradient-l-offset})
         );
 
         background-image: linear-gradient(