fix missing categories for flask tutorial (#5127)

Author: bene2k1

tutorials/flask/index.mdx

@@ -6,6 +6,7 @@ content:
   h1: Configuring Flask on Ubuntu Bionic Beaver
   paragraph: This page details how to install and configure Flask
 tags: Flask Ubuntu Bionic-Beaver
+categories:
   - instances
 dates:
   validation: 2025-04-08
@@ -301,4 +302,4 @@ Let's try and display random Scaleway catchphrases instead of always the same on
 
 <Lightbox src="scaleway-FlaskQuote.webp" alt="" />
 
-To learn more about Flask, refer to the official [Flask documentation](http://flask.pocoo.org/).
+To learn more about Flask, refer to the official [Flask documentation](http://flask.pocoo.org/).