doc/diffsum: fix spelling

5 jobs for diffrepro-improvements in 1 minute and 3 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Test
passed #185524
amd64 linux docker
test:flake8

00:00:17

passed #185523
amd64 linux docker
test:mypy

00:00:23

passed #185525
amd64 linux docker
test:pylint

00:00:30

passed #185526
amd64 linux docker
test:pytest

00:00:20

passed #185527
amd64 linux docker
test:respdiff

00:01:03