filestore.backup: use correct api reference link in documentation (#15380)

Author: BBBmau

mmv1/products/filestore/Backup.yaml

@@ -19,7 +19,7 @@ references:
   guides:
     'Official Documentation': 'https://cloud.google.com/filestore/docs/backups'
     'Creating Backups': 'https://cloud.google.com/filestore/docs/create-backups'
-  api: 'https://cloud.google.com/filestore/docs/reference/rest/v1/projects.locations.instances.backups'
+  api: 'https://cloud.google.com/filestore/docs/reference/rest/v1/projects.locations.backups'
 docs:
 base_url: 'projects/{{project}}/locations/{{location}}/backups'
 self_link: 'projects/{{project}}/locations/{{location}}/backups/{{name}}'