fix(docs): correct router variable name in custom middleware example (#348)

Replaced incorrect usage of 'router.Run(":8080")' with 'r.Run(":8080")' in the custom middleware example. 
The variable 'r' is initialized with gin.New() and should be used to start the server.
This change prevents potential runtime errors due to undefined 'router'.

Author: SnazzyNivesh522

src/content/docs/en/docs/examples/custom-middleware.md

@@ -36,7 +36,7 @@ func main() {
 	})
 
 	// Listen and serve on 0.0.0.0:8080
-	router.Run(":8080")
+	r.Run(":8080")
 }
 ```