chore: point submodule to own repo on old-ts-hero-v3 branch

The old TypeScript Hero v3 code now lives on the old-ts-hero-v3 branch
of this repo instead of referencing buehler/typescript-hero directly.

Author: JohannesHoppe

.gitmodules

@@ -1,3 +1,4 @@
 [submodule "tests/comparison/old-typescript-hero"]
 	path = tests/comparison/old-typescript-hero
-	url = https://github.com/buehler/typescript-hero.git
+	url = https://github.com/angular-schule/mini-typescript-hero.git
+	branch = old-ts-hero-v3