Fix command syntax for building with devOutput

Author: MarcusCaspeco

src/content/docs/en/guides/environment-variables.mdx

@@ -96,7 +96,7 @@ This allows you to run the dev server or build your site connecting to different
     npm run astro dev --mode staging
 
     # Build a site that connects to a "production" API with additional debug information
-    npm run astro build --devOutput
+    npm run astro build -- --devOutput
 
     # Build a site that connects to a "testing" API
     npm run astro build --mode testing