fix(docs-infra): dim line-numbers in code examples (#40802)

This commit ensures that the line-numbers in code examples appear
"dimmed" to make it clearer that they are not part of the actual code.

Based on the CSS rules, it seems that the intention was to make it so,
but the color was incorrectly set on the `<ol>` elements instead of
targeting `li::marker`.

Before: ![line-numbers before][1]
After: ![line-numbers after][2]

[1]: https://user-images.githubusercontent.com/8604205/106937191-23aeab80-6726-11eb-9178-bdc067fa3d09.png
[2]: https://user-images.githubusercontent.com/8604205/106937184-227d7e80-6726-11eb-81d1-f709537b6a56.png

PR Close #40802

aio/src/styles/2-modules/_code.scss

@@ -109,13 +109,17 @@ aio-code {
       ol.linenums {
         margin: 0;
         font-family: $main-font;
-        color: #B3B6B7;
+        color: lighten($mediumgray, 25%);
 
         li {
           margin: 0;
           font-family: $code-font;
           font-size: 90%;
           @include line-height(24);
+
+          &::marker {
+            color: lighten($mediumgray, 25%);
+          }
         }
       }