[css-forms-1] Fix syntax error in stylesheet (#13671)

Author: lukewarlow

css-forms-1/Overview.bs

@@ -845,7 +845,7 @@ select {
 ISSUE(12267): Refine meter, progress, switch and range input styling.
 
 ```css
-input[type="checkbox][switch],
+input[type="checkbox"][switch],
 input[type="range"] {
   display: inline-flex;
   position: relative;