Fixes Bug On alert_grouping field when Importing Services

[stmcallister]

When importing pagerduty_service resources the alert_grouping and alert_grouping_timeout` fields were not getting populated by the imported service. This PR fixes that issue.

Test results

TF_ACC=1 go test -run "TestAccPagerDutyService_*" ./pagerduty -v -timeout 120m

=== RUN   TestAccPagerDutyServiceIntegration_import
--- PASS: TestAccPagerDutyServiceIntegration_import (8.46s)
=== RUN   TestAccPagerDutyService_import
--- PASS: TestAccPagerDutyService_import (7.11s)
=== RUN   TestAccPagerDutyServiceWithIncidentUrgency_import
--- PASS: TestAccPagerDutyServiceWithIncidentUrgency_import (7.57s)
=== RUN   TestAccPagerDutyServiceIntegration_Basic
--- PASS: TestAccPagerDutyServiceIntegration_Basic (12.46s)
=== RUN   TestAccPagerDutyServiceIntegrationGeneric_Basic
--- PASS: TestAccPagerDutyServiceIntegrationGeneric_Basic (11.10s)
=== RUN   TestAccPagerDutyService_Basic
--- PASS: TestAccPagerDutyService_Basic (12.77s)
=== RUN   TestAccPagerDutyService_AlertGrouping
--- PASS: TestAccPagerDutyService_AlertGrouping (10.62s)
=== RUN   TestAccPagerDutyService_BasicWithIncidentUrgencyRules
--- PASS: TestAccPagerDutyService_BasicWithIncidentUrgencyRules (12.43s)
=== RUN   TestAccPagerDutyService_FromBasicToCustomIncidentUrgencyRules
--- PASS: TestAccPagerDutyService_FromBasicToCustomIncidentUrgencyRules (9.85s)
=== RUN   TestAccPagerDutyService_SupportHoursChange
--- PASS: TestAccPagerDutyService_SupportHoursChange (9.58s)
PASS
ok  	github.com/terraform-providers/terraform-provider-pagerduty/pagerduty	104.316s

[heimweh]

Nice! This LGTM 👍