Turn off pre-commit's automated multiprocessing

[peterjc]

Since the doc8 output includes a summary footer, this appears multiple times if pre-commit has run multiple copies of doc8 in parallel.

(This change would also be recommended if doc8 had its own multi-threading support, but I am unsure if that is the case)

[peterjc]

An alternative workaround for the end user:

-   repo: https://github.com/PyCQA/doc8
    rev: 0.8.1
    hooks:
    -   id: doc8
        args: [--quiet]