Bug 1565149 [wpt PR 17748] - [docs] Correct typo in example of user s…

…tyles, a=testonly

Automatic update from web-platform-tests
[docs] Correct typo in example of user styles

`userstyle` and `nouserstyle` are referenced as CSS class names in this
document, so they should be declared as such.

--

wpt-commits: 0e95bf3183c7358f3bb428f6f1cc3db5e86b1e79
wpt-pr: 17748

testing/web-platform/tests/docs/writing-tests/css-user-styles.md

@@ -25,12 +25,12 @@ should not be applied.
 Harness style sheet rules:
 
 ``` css
-#userstyle
+.userstyle
 {
     color: green;
     display: none;
 }
-#nouserstyle
+.nouserstyle
 {
     color: red;
     display: none;