bookstack: fix syntax in option example (#381392)

          OIDC_NAME = "MyLogin";

          OIDC_DISPLAY_NAME_CLAIMS = "name";

          OIDC_CLIENT_ID = "bookstack";

          OIDC_CLIENT_SECRET = {_secret = "/run/keys/oidc_secret"};

          OIDC_CLIENT_SECRET = {_secret = "/run/keys/oidc_secret";};

          OIDC_ISSUER = "https://keycloak.example.com/auth/realms/My%20Realm";

          OIDC_ISSUER_DISCOVER = true;

        }