Explicitly mention 'for' loop limit in step 5 of "Turtle Square"

[ganicke]

Mention the loop limit for the index in the for loop of step 5. Seems that some users may miss this after dropping in the loop block.

This could get preset as a custom toolbox block if blockconfig worked in this target?

```blockconfig.global
for(let index = 0; index <= 3; index++) {}
```

Closes #5841