fix broken border-radius helper example

In https://github.com/primer/primer/pull/545/files#diff-1d59c9367289419da3ed2963ed24e7beR132, the docs introduced for the `padding-*-*`  helpers omitted the radius size dimension.

cc @shawnbot

modules/primer-utilities/docs/borders.md

@@ -130,17 +130,17 @@ Use the following utilities to add or remove rounded corners: `rounded-0` remove
 You can also add rounded corners to each edge (top, right, bottom, left) with the following utilities:
 
 ```html
-<div class="border rounded-top mb-2">
-  .rounded-top
+<div class="border rounded-top-1 mb-2">
+  .rounded-top-1
 </div>
-<div class="border rounded-right mb-2">
-  .rounded-right
+<div class="border rounded-right-1 mb-2">
+  .rounded-right-1
 </div>
-<div class="border rounded-bottom mb-2">
-  .rounded-bottom
+<div class="border rounded-bottom-1 mb-2">
+  .rounded-bottom-1
 </div>
-<div class="border rounded-left mb-2">
-  .rounded-left
+<div class="border rounded-left-1 mb-2">
+  .rounded-left-1
 </div>
 ```