fixes "@source"-call to actually exclude the vendor-dir (#2497)

Author: friedrichweise

content/docs/3_cookbook/0_frontend/0_kirby-meets-tailwindcss/cookbook-recipe.txt

@@ -102,7 +102,7 @@ The above `tailwind.css` file is everything that is needed to run Tailwind CSS,
 @import "tailwindcss";
 
 /* Exclude the 'vendor' directory from beeing scanned */
-@source "../vendor";
+@source not "../vendor";
 
 /* Adjusting variables and adding custom styles */
 @theme {