Update: Silo example

[blaisb]

Description

The silo examples works just fine. I just fixed some information within the example to make it slightly easier to read and fixed some typos there and there.

###Comment

Reading this example made me realize that the way we manage floating walls is kinda lame, so I have opened an issue #1360 regarding this.

Code related list:

• All in-code documentation related to this PR is up to date (Doxygen format)
• Copyright headers are present and up to date
• Lethe documentation is up to date
• The branch is rebased onto master
• Code is indented with indent-all and .prm files (examples and tests) with prm-indent
• If parameters are modified, the tests and the documentation of examples are up to date
• Changelog (CHANGELOG.md) is up to date if the refactor affects the user experience or the codebase

Pull request related list:

• No other PR is open related to this refactoring
• Labels are applied
• There are at least 2 reviewers (or 1 if small feature) excluding the responsible for the merge
• If this PR closes an issue or is related to a project, it is linked in the "Projects" or "Development" section
• If any future works is planned, an issue is opened
• The PR description is cleaned and ready for merge

[lpsaavedra]

Found some minor things to improve