Fix missing space in warning message (#6340)

src/gbm/gbtree.cc

@@ -81,7 +81,7 @@ void GBTree::Configure(const Args& cfg) {
     LOG(WARNING) << "DANGER AHEAD: You have manually specified `updater` "
         "parameter. The `tree_method` parameter will be ignored. "
         "Incorrect sequence of updaters will produce undefined "
-        "behavior. For common uses, we recommend using"
+        "behavior. For common uses, we recommend using "
         "`tree_method` parameter instead.";
     // Don't drive users to silent XGBOost.
     showed_updater_warning_ = true;