Merge pull request #6450 from ipfs/fix/submodule-use-http

gitmodules: use https so we don't need an ssh key

.gitmodules

 [submodule "assets/dir-index-html"]
 	path = assets/dir-index-html
-	url = git@github.com:ipfs/dir-index-html
+	url = https://github.com/ipfs/dir-index-html.git