fix: move to nunjucks template syntax for issue URL in exercise-started.md (#77)

Author: FidelusAleksander

markdown-templates/readme/exercise-started.md

@@ -8,7 +8,7 @@ Mona here. I'm done preparing your exercise. Hope you enjoy! 💚
 
 Remember, it's self-paced so feel free to take a break! ☕️
 
-[![](https://img.shields.io/badge/Go%20to%20Exercise-%E2%86%92-1f883d?style=for-the-badge&logo=github&labelColor=197935)]({{{ issue_url }}})
+[![](https://img.shields.io/badge/Go%20to%20Exercise-%E2%86%92-1f883d?style=for-the-badge&logo=github&labelColor=197935)]({{ issue_url }})
 
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