add url in comment

src/client/styles/scss/_override-bootstrap.scss

@@ -116,7 +116,8 @@ label {
   font-weight: 700;
 }
 
-// disabled button (reproduction from bootstrap3)
+// disabled button (reproduction from bootstrap3.)
+// see https://cccabinet.jpn.org/bootstrap4/components/buttons#disabled-state
 .btn.disabled,
 .btn[disabled],
 fieldset[disabled] .btn {