Update cron documentation to include Spring formatting in "Importing Services & APIs"

[yada]

Describe the bug

In doc: https://github.com/microcks/microcks.io/edit/master/content/documentation/guides/usage/importing-content.md

Just replace:

The value should be set to a valid [CRON expression](https://en.wikipedia.org/wiki/Cron#CRON_expression); the default is every 2 hours.

by

The value should be set to a valid [CRON expression](https://spring.io/blog/2020/11/10/new-in-spring-5-3-improved-cron-expressions); the default is every 2 hours.

Attach any resources that can help us understand the issue.

This was discussed a long time ago and has not been pushed... time to clean it up ;)
ref: microcks/microcks.github.io#6