Examples update for ball-packing and rectangular-hopper

[OresteMarquis]

Description

Update of the .prm and .rst files of the ball-packing and rectangular-hopper examples for the Lethe 1.0 release. The rectangular hopper gave similar curves visually, but the mass flow rate is different than the one stated in the documentation (85.17 vs 84.94 & 90.12 vs 85.09 g/s). Nonetheless, I feel that those results are close enough that it is not necessary to change the example documentation for it.

Miscellaneous (will be removed when merged)

Checklist (will be removed when merged)

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Code related list:

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

Pull request related list:

• 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