Remove erroneous trailing slash (#1963)

Author: wongjn

src/docs/adding-custom-styles.mdx

@@ -210,7 +210,7 @@ While Tailwind is designed to handle the bulk of your styling needs, there is no
 
 ```css
 /* [!code filename:CSS] */
-@import "tailwindcss/";
+@import "tailwindcss";
 
 /* [!code highlight:4] */
 .my-custom-style {