Update middleware.md

[mrpritchett]

Incorrect path for middleware docuemntation.

[drbyte]

Thanks for your contribution.
You've submitted a PR to the v5 branch where the docs reflect the namespace that was used in v5 and older.
However, the current release is v6, and the v6 documentation shows the correct new namespace.

Changing the docs according to this PR would make them inconsistent with the actual package code.