Make submodules track main branch.

.gitmodules

@@ -1,6 +1,8 @@
 [submodule "axisregistry"]
 	path = axisregistry
 	url = https://github.com/googlefonts/axisregistry
+	branch = main
 [submodule "lang"]
 	path = lang
 	url = https://github.com/googlefonts/lang
+	branch = main