Fix the OP_ID of OpAssignGroupNodes

Commit 96276ae7 added the above opcode, but forgot to update the OP_ID
per our previous mailing list discussions. This patch makes the OP_ID
be consistent with the other IDs (OP_$object_$action).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>

lib/opcodes.py

@@ -1029,7 +1029,7 @@ class OpAddGroup(OpCode):
 
 class OpAssignGroupNodes(OpCode):
   """Assign nodes to a node group."""
-  OP_ID = "OP_ASSIGN_NODES"
+  OP_ID = "OP_GROUP_ASSIGN_NODES"
   OP_DSC_FIELD = "group_name"
   OP_PARAMS = [
     _PGroupName,