launchpad/tasks: Fix ping task function name
An exception is raised when registering task types in scheduler database otherwise.
swh.core.api.RemoteException: <RemoteException 500 NotNullViolation: ['null value in column "description" violates not-null constraint\nDETAIL: Failing row contains (ping, null, swh.lister.launchpad.tasks.ping, 1 day, 1 day, 1 day, 1, null, null, null).\n']>
I should have tested one more time in docker before landing !197 (closed), my bad.
Migrated from D4964 (view on Phabricator)