Fix yaml indention of GCS secretName
What does this MR do?
Fix the indention of the runners.cache.secretName
key.
Why was this MR needed?
I copied it blindly and the cache didn't work because of this.
What's the best way to test this MR?
Deploy Gitlab runner with GCS cache and config copied from the relevant docs section.
It would be good if the yaml parser would have caught this when trying to parse invalid data. That is something that should be fixed imo.