formatting of usage

Former-commit-id: 3a98b86436a6a840f5a3822218aff75b174b1a86

include/igl/viewer/Viewer.cpp

@@ -353,7 +353,7 @@ namespace viewer
   :       Toggle face labels)"
 #endif
 );
-    std::cout<<usage;
+    std::cout<<usage<<std::endl;
 #endif
   }