docs: override.styl to style.styl for custom page class (#939)

Author: thiemo

docs/default-theme-config/README.md

@@ -492,7 +492,7 @@ pageClass: custom-page-class
 Then you can write CSS targeting that page only:
 
 ``` css
-/* .vuepress/override.styl */
+/* .vuepress/style.styl */
 
 .theme-container.custom-page-class {
   /* page-specific rules */