icinga: fix a typo on the graphql host declaration
Fix this error:
Sep 12 14:21:45 pergamon safe-reload[3710886]: [2022-09-12 14:21:45 +0000] critical/config: Error: Validation failed for object 'graphql.staging.swh.network!Software Heritage Staging Graphql Instance' of type 'Service'; Attribute 'host_name': Object 'graphql.staging.swh.network' of type 'Host' does not exist.
Related to T4135