Yes, but so is the PR shown in the article. You're not going to get a diff that's less than one line unless you are using something besides the typical diff and patch tools.

My smallest PR was adding a missing executable file permission.

I think that wins, since presumably it was smaller than a one-byte change. I guess the smallest would be a single-bit file mode change, maybe?

Only the user permission? Or group and others also?

In terms of file contents, I have a bugfix commit somewhere that just adds a single newline to a config file (no line deleted in the diff). We were using something that had its own own custom parser for what looked like an INI file, and we managed to find an edge case where the wrong number of blank lines would break it.