[docs]: write_files "source" key is not a list

# Documentation request

https://docs.cloud-init.io/en/latest/reference/modules.html#write-files

Go to "Config schema" tab, look at the description of the "source" attribute. It says:

> source: (object) Optional specification for content loading from an arbitrary URI. Each object in source list supports the following keys:

However, source is not a list, it's a single object. Suggested replacement:

> source: (object) Optional specification for content loading from an arbitrary URI. The source object supports the following keys:

Open the linked write_files documentation and inspect the Config schema tab's description of the source attribute. Update the wording so it describes source as a single object, then verify the rendered documentation matches the suggested replacement.

Written by the indexing model from the issue text.