Bump Coq version in README.

2 jobs for master in 6 minutes and 39 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
failed #26077
fp
build-iris.dev

00:06:39

failed #26029
fp
build-iris.dev

00:05:59

 
Name Stage Failure
failed
build-iris.dev Build
Makefile.coq:326: recipe for target 'all' failed
make[1]: *** [all] Error 2
Makefile:6: recipe for target 'all' failed
make: *** [all] Error 2

real 1m41.294s
user 2m18.704s
sys 0m6.004s
ERROR: Job failed: exit code 1