Merge pull request #432 from ZJU-SEL/fix-exist

fix minor typo
diff --git a/libcontainer/error.go b/libcontainer/error.go
index c51a364..aa59d2a 100644
--- a/libcontainer/error.go
+++ b/libcontainer/error.go
@@ -45,7 +45,7 @@
 	case ContainerNotRunning:
 		return "Container is not running"
 	case ConsoleExists:
-		return "Console exist for process"
+		return "Console exists for process"
 	default:
 		return "Unknown error"
 	}