fix(firestore): create doc before updating (#1828)
fix(firestore): create doc before updating (#1828)

After #1810, the test for this snippet was failing because the
snippet updated a doc which had not yet been created. This adds
the doc creation to the same function (but outside the region tag).

Fixes #1826
GoogleCloudPlatform/golang-samples,GoogleCloudPlatform/golang-samples,GoogleCloudPlatform/golang-samples