added correct borders to the ribbon start class (#1936)

src/scss/ui/_ribbons.scss

@@ -87,6 +87,7 @@
 .ribbon-start {
   right: auto;
   left: calc(-1 * var(--#{$prefix}ribbon-margin));
+  border-radius: 0 var(--#{$prefix}ribbon-border-radius) var(--#{$prefix}ribbon-border-radius) var(--#{$prefix}ribbon-border-radius);
 
   &:before {
     top: auto;