fix typo in README.md

7 jobs for master in 21 minutes and 28 seconds (queued for 46 seconds)
Status Job ID Name Coverage
  Build
passed #22303
fp
build-coq.8.7.1

00:05:22

passed #22302
fp
build-coq.8.7.2

00:05:28

passed #22301
fp-timing
build-coq.8.8.0

00:05:02

passed #22300
fp
build-coq.8.8.1

00:06:26

passed #22299
fp
build-coq.8.8.2

00:06:27

passed #22298
fp-timing
build-coq.8.9.dev

00:05:47

passed #22297
fp
build-coq.dev

00:20:56