Fix 'This flag is used as a command name. Bad line break or missing [ .. ]?' issue in activity.sh

[CodeFactor](https://www.codefactor.io/repository/github/ccdd13/github-activity-bash-script) found an issue: This flag is used as a command name. Bad line break or missing [ .. ]?

It's currently on:
[activity.sh:53
](https://www.codefactor.io/repository/github/ccdd13/github-activity-bash-script/source/main/activity.sh#L53)