Use DB_CREDENTIALS instead of GCP_CREDENTIALS in files example

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Sprite

reference/configuration.html.markerb

@@ -1241,10 +1241,10 @@ Write a secret to a file:
 ```toml
 [[files]]
   guest_path = "/app/credentials.json"
-  secret_name = "GCP_CREDENTIALS"
+  secret_name = "DB_CREDENTIALS"
 ```
 
-This writes the (base64-decoded) content of the `GCP_CREDENTIALS` secret to `/app/credentials.json`. Set the secret with: `fly secrets set GCP_CREDENTIALS=$(base64 < credentials.json)`.
+This writes the (base64-decoded) content of the `DB_CREDENTIALS` secret to `/app/credentials.json`. Set the secret with: `fly secrets set DB_CREDENTIALS=$(base64 < credentials.json)`.
 
 You can optionally restrict which Machine(s) contain the file using the [processes](#the-processes-section) field. For example: