Change the word "mandatory" to required field

The wording change makes sense.

This should also be added to the module's ReadMe file.

I've been thinking it should also be possible to make the module alert you if the field is misconfigured as a required field, but that should be a separate issue.

I have updated on the project page.

I changed the text to:

Do not set the Scheduled Publish field as a required field. Since it's value will be set to "null", after cron has made the transition, the field will fail to update if it is required.

The sentence starting "Since" felt incomplete.

I have updated the text in the module page.

I have checked and merged @cindy_codediaries' change to the Readme file.

Automatically closed - issue fixed for 2 weeks with no activity.