docs: fix typo in pre-wp

1 job for master in 4 minutes and 10 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #655
coq
buildjob

00:04:10

 
Name Stage Failure
failed
buildjob Test There is an unknown failure, please try again
tests/joining_existentials (user: 9.28 mem: 521292 ko)
$ make verify
make -f Makefile.coq verify
make[1]: Entering directory '/builds/FP/iris-coq'
make[1]: *** No rule to make target 'verify'. Stop.
make: *** [verify] Error 2
make[1]: Leaving directory '/builds/FP/iris-coq'
Makefile:4: recipe for target 'verify' failed
ERROR: Build failed: exit code 1