fix error on swarm constraint exemple (#749)

Constraints for Swarm scheduling are defined as an **environment** option, according to the example in the section *Manual scheduling*, and not as a **label** option

compose/swarm.md

@@ -82,15 +82,15 @@ all three services end up on the same node:
         image: foo
         volumes_from: ["bar"]
         network_mode: "service:baz"
-        labels:
+        environment:
           - "constraint:node==node-1"
       bar:
         image: bar
-        labels:
+        environment:
           - "constraint:node==node-1"
       baz:
         image: baz
-        labels:
+        environment:
           - "constraint:node==node-1"
 
 ### Host ports and recreating containers
@@ -169,4 +169,4 @@ them.
       - "affinity:image==redis"
 
 For the full set of available filters and expressions, see the [Swarm
-documentation](/swarm/scheduler/filter.md).
+documentation](/swarm/scheduler/filter.md).