Update QRCard qr's z-index to be less arbitrary (#14274)

Author: chrstinalin

src/amo/components/QRCard/styles.scss

@@ -34,7 +34,7 @@
 
 .QRCard-qr-wrapper {
   position: absolute;
-  z-index: 10;
+  z-index: 2;
   width: 35%;
   min-width: 76px;
   transform: translate(-2.5px, -8px);