point to hashicorp's cronexpr (#18510)

point to hashicorp's cronexpr

website/content/docs/job-specification/periodic.mdx

@@ -42,7 +42,7 @@ consistent evaluation when Nomad spans multiple time zones.
   launched at. The job runs at the next earliest time that matches any of the
   expressions. Supports predefined expressions such as `@daily` and
   `@weekly`. Refer to [the
-  documentation](https://github.com/gorhill/cronexpr#implementation) for full
+  documentation](https://github.com/hashicorp/cronexpr#implementation) for full
   details about the supported cron specs and the predefined expressions.
   Conflicts with `cron`.