node-executors-help: correct agent to node

This screen is in the nodes view and can be reached for:
/computer/(master)/configure

The help applies to all nodes (agents and master) and therefore this text
should say node.

core/src/main/resources/hudson/model/Node/help-numExecutors.html

@@ -1,6 +1,6 @@
 <div>
   The maximum number of concurrent builds that Jenkins may perform on this
-  agent.
+  node.
   <p>
   A good value to start with would be the number of CPU cores on the machine.
   Setting a higher value would cause each build to take longer, but could