fix name of recv_strengthen to recv_weaken

1 job for master
Name Stage Failure
buildjob Test There is an unknown failure, please try again
make[1]: *** [barrier/specification.vo] Error 1
make[1]: *** Waiting for unfinished jobs....
Closed under the global context
barrier/client (user: 7.68 mem: 542752 ko)
make[1]: Leaving directory '/builds/FP/iris-coq'
Makefile:4: recipe for target 'all' failed
make: *** [all] Error 2

ERROR: Build failed with: exit code 1