Fix stale provider binary name in .gitignore

The rebrand renamed the Terraform provider binary to
terraform-provider-cairnobs (terraform/main.go, and the build command in
terraform/README.md), but .gitignore still listed the old
terraform-provider-sentry. The pattern no longer matched, so a locally
built provider binary would have shown up as untracked and could have
been committed by a `git add -A`.

Verified by building the provider and confirming git check-ignore now
matches it.

.gitignore

@@ -7,7 +7,7 @@ search/target/
 /ingest/ingest
 /api/api
 /cli/cairnobsctl
-/terraform/terraform-provider-sentry
+/terraform/terraform-provider-cairnobs
 /alerting/alerting
 /hack/windows-fixture/windows-fixture
 /hack/benchmark-fixture/benchmark-fixture