Docs: fix background-blend-mode CSS value (#40931)

Author: MohamadSalman11

site/assets/scss/_masthead.scss

@@ -86,7 +86,7 @@
   padding: 1rem;
   color: rgba(var(--bg-rgb), 1);
   background-color: rgba(var(--bg-rgb), .1);
-  background-blend-mode: multiple;
+  background-blend-mode: multiply;
   @include border-radius(1rem);
   mix-blend-mode: darken;