Fix two errors in the deamon_plugin documentation

[benoit5672]

Change documentation
pre-install.sh script was wrongly documented as post-install.sh
In the daemon_start() function, the condition to raise an error was wrong (I <= 30 instead of 20)

these changes has been already discussed with Mips.

Description

Suggested changelog entry

Related issues/external references

Fixes #

Types of changes

• Bug fix (non-breaking change which fixes)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
  • This change is only breaking for integrators, not for external standards or end-users.
• Documentation improvement

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the [La ligne directrice pour contribuer à ce projet / Contribution guidelines for this project).
• I grant the project the right to include and distribute the code under the GNU.
• I have added tests to cover my changes.
• I have verified that the code complies with the projects coding standards.
• [Required for new sniffs] I have added MD documentation for the sniff.

[benoit5672]

@Mips2648 , this is the PR we discuss about daemon_plugin documentation.
Thanks