Update backup and restore documentation

[vmonegatto]

Just updating the documentation due to typos or modification in the GUI and in some of the default directories.

Pull Request Template

Description

Please include a short resume of the changes and what is the purpose of PR. Any relevant information should be added to help:

• QA Team (Quality Assurance) with tests.
• reviewers to understand what are the stakes of the pull request.

Fixes
This request fixes a number of typos and is updated with some of the new "default" directories used for backup and plugins.

Type of change

• Patch fixing an issue (non-breaking change)
• New functionality (non-breaking change)
• Breaking change (patch or feature) that might cause side effects breaking part of the Software
• Updating documentation (missing information, typo...)

Target serie

• 2.8.x
• 18.10.x
• 19.04.x
• 19.10.x
• 20.04.x (master)

[kduret]

Since documentation is not hosted in the repo anymore, I close this PR.
If some changes are needed, plus make a PR on https://github.com/centreon/centreon-documentation