[CSGI-2303] Hotfix - Alert grouping parameters input validation broken

[imjaroiswebdev]

Closes #778

Test cases extended...

$ make testacc TESTARGS="-run TestAccPagerDutyService_AlertContentGrouping"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run TestAccPagerDutyService_AlertContentGrouping -timeout 120m
?       github.com/terraform-providers/terraform-provider-pagerduty     [no test files]
=== RUN   TestAccPagerDutyService_AlertContentGrouping # 👈 Test case extended to cover regression
--- PASS: TestAccPagerDutyService_AlertContentGrouping (66.24s)
=== RUN   TestAccPagerDutyService_AlertContentGroupingIntelligentTimeWindow
--- PASS: TestAccPagerDutyService_AlertContentGroupingIntelligentTimeWindow (19.25s)
PASS
ok      github.com/terraform-providers/terraform-provider-pagerduty/pagerduty   86.107s

$ make testacc TESTARGS="-run TestAccPagerDutyService_FormatValidation"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run TestAccPagerDutyService_FormatValidation -timeout 120m
?       github.com/terraform-providers/terraform-provider-pagerduty     [no test files]
=== RUN   TestAccPagerDutyService_FormatValidation # 👈 Test case extended to cover regression
--- PASS: TestAccPagerDutyService_FormatValidation (34.79s)
PASS
ok      github.com/terraform-providers/terraform-provider-pagerduty/pagerduty   35.488s

[imjaroiswebdev]

Proceeding to merge in order to promptly patch #778