Fix plugin import path in README.md (#382)

Author: man-shar

README.md

@@ -30,7 +30,7 @@ Then add the plugin to your main `style.css` file:
 
 ```diff
   @import "tailwindcss";
-+ @plugin "tailwindcss/typography";
++ @plugin "@tailwindcss/typography";
 ```
 
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