fix: close file in the generateAtDir func

[testwill]

• PR Description
  close file in the generateAtDir func
• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• Docs (specifically docs/Config.md) have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[stefanhaller]

The change looks obviously right, but I'm curious what your reason was for making this change. Did you actually have a problem that is fixed by this? (Like, were you running out of file handles? Seems unlikely to me.) I find it helpful to mention this kind of information in the PR description and/or commit message.

[jesseduffield]

I also am simultaneously grateful for the PR and confused at the motivation. @testwill I see that you've got at least 600 repositories forked and most of them, from what I've seen, are forked for the purpose of raising a PR that fixes a typo or closes a file handle.

I'm curious: did you raise this PR yourself or was this automated?

[testwill]

raise this PR myself. typo can use misspell, but not very precise. No tool can detect closing file handle,according to my understanding.