ci: fix dependabot config

.github/dependabot.yml

@@ -1,11 +1,11 @@
 # To get started with Dependabot version updates, you'll need to specify which
 # package ecosystems to update and where the package manifests are located.
 # Please see the documentation for all configuration options:
-# https://docs.github.com/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
+# https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
 
 version: 2
 updates:
-  - package-ecosystem: "" # See documentation for possible values
+  - package-ecosystem: "cargo" # See documentation for possible values
     directory: "/" # Location of package manifests
     schedule:
       interval: "weekly"