Minor corrections to turbulent taylor couette example

[lpsaavedra]

Description

Default variables were removed from prm file and the linear solver parameters were simplified. The results remain the same. Since the example is pretty recent only minor corrections were done in the documentation page.

Checklist (will be removed when merged)

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Code related list:

• [x] Lethe documentation is up to date
• [x] Copyright headers are present and up to date
• [x] The branch is rebased onto master
• [x] Code is indented with indent-all and .prm files (examples and tests) with prm-indent
• [x] Links are added to parent .rst files
• [x] The example is following the standard format

Pull request related list:

• [x] Labels are applied
• [ ] There are at least 2 reviewers (or 1 if small feature) excluding the responsible for the merge
• [x] If this PR closes an issue or is related to a project, it is linked in the "Projects" or "Development" section
• [x] If any future works is planned, an issue is opened
• [x] The PR description is cleaned and ready for merge