.gitmodules: update smmap url

Github.com is currenly redirecting the smmap url:
https://github.com/Byron/smmap.git
to:
https://github.com/gitpython-developers/smmap

For correctness we update to the url directly

Author: RandyMcMillan

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "smmap"]
 	path = gitdb/ext/smmap
-	url = https://github.com/Byron/smmap.git
+	url = https://github.com/gitpython-developers/smmap.git