Use upstream lwtnn submodule URL (#340)

Author: sbein

.gitmodules

@@ -15,4 +15,4 @@
 	url = https://github.com/Cyan4973/xxHash.git
 [submodule "lwtnn"]
 	path = lwtnn
-	url = https://github.com/sbein/lwtnn.git
+	url = https://github.com/lwtnn/lwtnn.git