Turn off pre-commit's automated multiprocessing

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

Author: peterjc

.pre-commit-hooks.yaml

@@ -9,3 +9,4 @@
   entry: python -m doc8
   language: python
   files: \.rst$
+  require_serial: true