enable multiprocessing for pyspelling

Pyspelling has a new option to run jobs in parallel. See
facelessuser/pyspelling#182

This doesn't make much difference (if at all) for the starter
pack, but it's considerably faster for large documentation sets.

Signed-off-by: Ruth Fuchss <ruth.fuchss@canonical.com>

.sphinx/spellingcheck.yaml

@@ -1,3 +1,4 @@
+jobs: 8
 matrix:
 - name: rST files
   aspell: