Update README.md (SASS deprecation)

Updated SASS code to not used deprecated methods.

Author: ffxsam

README.md

@@ -247,18 +247,20 @@ and so on.
 Better yet, here is a SASS code snippet which can make life much easier (Thanks to @ascendantofrain, [#81](https://github.com/gridstack/gridstack.js/issues/81) and @StefanM98, [#868](https://github.com/gridstack/gridstack.js/issues/868)) and you can use sites like [sassmeister.com](https://www.sassmeister.com/) to generate the CSS for you instead:
 
 ```sass
-@use "sass:math";
 .grid-stack > .grid-stack-item {
+  $gridstack-columns: 2;
 
-  $gridstack-columns: 12;
-
-  min-width: 100%/$gridstack-columns;
+  min-width: calc(100% / $gridstack-columns);
 
   @for $i from 0 through $gridstack-columns {
-    &[gs-w='#{$i}'] { width: (100% / $gridstack-columns) * $i; }
-    &[gs-x='#{$i}'] { left: (100% / $gridstack-columns) * $i; }
+    &[gs-w='#{$i}'] {
+      width: (calc(100% / $gridstack-columns)) * $i;
+    }
+    &[gs-x='#{$i}'] {
+      left: (calc(100% / $gridstack-columns)) * $i;
+    }
   }
-} 
+}
 ```
 
 you can also use the SASS [src/gridstack-extra.scss](https://github.com/gridstack/gridstack.js/tree/master/src/gridstack-extra.scss) included in NPM package and modify to add more columns.