Social-Feed Plugin: Running a Cronjob

I am using this social-feed plugin and it ask to run a line as a cronjob to get the posts locally.
https://github.com/moduleon/grav-plugin-social-feed

'/15 * php /path/to/my/site/bin/plugin social-feed fetch:posts'

It doesn't seem to run it. The error is: 'FATAL: Must be run from ROOT directory of Grav!'

Any ideas on how to rewrite this for cronjob? Running manually works. Just not as a cron .

Here is what worked for me.

/15 * cd /path/to/grav/site/; php bin/plugin social-feed fetch:posts > /dev/null 2>&1