Minor documentation error

[schwobaseggl]

Summary:

In the README it states that the crontab 30 * * * * corresponds to "execute every 30 minutes".

Detailed information

This is, of course, wrong. It means "execute at minute 30 of every hour".

[cclauss]

Agreed:

• https://crontab.guru/#30_*_*_*_* is At minute 30. (every hour, every day, every month)
• https://crontab.guru/#*/30_*_*_*_* is Every 30th minutes.

Fixed in #689

• Update README.rst - Crontab effect description #689