docs: Update correct locations for Assistant contexts (#17049)

The location of Assistant contexts in the docs is wrong. The actual path
for the contexts is defined
[here](https://github.com/zed-industries/zed/blob/1eec601afb8cb0b2eafe111de1a505049202e19f/crates/paths/src/paths.rs#L159-L171).

Release Notes:

- N/A