Update flow around sphere example

[lpsaavedra]

Description

The example needed to be verified. The new boundary condition was added to the schematic of the case, minor typos were corrected, some sentences were rephrased, and the initial refinement parameter was missing from one of the parameter files. After the changes all the results presented in the doc page correspond to the ones obtained with the parameter files given in the examples folder.

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] The PR description is cleaned and ready for merge