chore: fix typo in route config validation error message

[SungJin1212]

This PR fixes a typo in the route config validation error message.

[grobinson-grafana]

Can you also fix this in the tests?

--- FAIL: TestWildcardGroupByWithOtherGroupByLabels (0.00s)
    config_test.go:309: 
        expected:
        "cannot have wildcard group_by (`...`) and other other labels at the same time"
        got:
        "cannot have wildcard group_by (`...`) and other labels at the same time"