docs: use more consistent and accurate glob for ignoring subfolders

[Weetbix]

Description:
This PR updates the readme to cover this use case a bit more accurately.

Currently, in the example touching files in /src/docs/some/subfolder/file.txt would still add the source label. This is probably not the behaviour most people are interested in, and is a bit different from what the comment says:

Add 'source' label to any change to src files within the source dir EXCEPT for the docs sub-folder

By switching to **/* we can recursively match files inside the docs folder, not just files directly in docs.

Related issue:

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.