Update install.md with 'transcodes' param in default settings.yml file

[nicolas-crauc]

install.md updated to include 'transcodes' param in default settings.yml

Please describe the purpose of this Pull Request.

For new Sandbox app/role documentation, please confirm you have completed the following tasks:

• [] App documentation page created
• [] markdownlint reports no errors on edited page(s) - use https://dlaa.me/markdownlint/ if your IDE does not have this built in
• [] Reference added in docs/sandbox/index.md
• [] Reference added in mkdocs.yml nav menu

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[maximuskowalski]

I think the default is actually /mnt/local/transcodes