Close the file after reading

[NobWow]

the file must be closed otherwise on Unix systems it creates an overflow

[appgurueu]

Technically garbage collection should eventually close the file:

Closes file. Note that files are automatically closed when their handles are garbage collected, but that takes an unpredictable amount of time to happen.

But it's definitely a code smell that I forgot to close it here, and as the reference manual states this can't be relied on.

Thanks for your PR!

[appgurueu]

Now that I see what I'm doing there: I should probably have avoided this altogether by using io.lines.