pid_limit -> pids_limit

"pids_limit" is the correct form, added in Docker Compose spec 2.1:
docker/compose@6a151aa

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

spec.md

@@ -1432,9 +1432,9 @@ be within [-1000,1000] range.
 `pid` sets the PID mode for container created by the Compose implementation.
 Supported values are platform specific.
 
-### pid_limit
+### pids_limit
 
-`pid_limit` tunes a container’s PIDs limit. Set to -1 for unlimited PIDs.
+`pids_limit` tunes a container’s PIDs limit. Set to -1 for unlimited PIDs.
 
 ```yml
 pids_limit: 10