Fix dependabot ecosystem value.

Author: davidlehn

.github/dependabot.yml

@@ -2,7 +2,7 @@ version: 2
 updates:
   # Update specifications' submodules
   # https://docs.github.com/en/code-security/dependabot/working-with-dependabot/dependabot-options-reference
-  - package-ecosystem: "submodules"
+  - package-ecosystem: "gitsubmodule"
     directory: "/"
     schedule:
       interval: "weekly"