Invalid mermaid arrow

[agusdmb]

          This is not valid mermaid.

it should be:
mod_c ..> mod_a
as documented here:
https://mermaid.js.org/syntax/classDiagram.html#defining-relationship

github renders mermaid files, and if we try to open this one we get an error, see:
https://github.com/pylint-dev/pylint/blob/ffe1e714a3cfe83b5a627ade6d723489f2e10621/tests/pyreverse/functional/package_diagrams/type_check_imports/packages.mmd

Originally posted by @agusdmb in #8824 (comment)