doc(pubsub): add region tag for subscription with filter

[coryan]

Use the existing example, it does what we want it just needed the right
region tags.

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[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• google/cloud/pubsub/samples/samples.cc:309, tag pubsub_create_subscription_with_filter

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[codecov]

Codecov Report

Merging #8326 (fbeb2e4) into main (79e9bdf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #8326   +/-   ##
=======================================
  Coverage   94.97%   94.97%           
=======================================
  Files        1345     1345           
  Lines      119817   119817           
=======================================
  Hits       113794   113794           
  Misses       6023     6023           

Impacted Files	Coverage Δ
google/cloud/pubsub/samples/samples.cc	92.10% <ø> (-0.16%)	⬇️
google/cloud/internal/async_rpc_details.h	94.44% <0.00%> (-5.56%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	89.04% <0.00%> (-0.71%)	⬇️
google/cloud/completion_queue_test.cc	96.95% <0.00%> (-0.20%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.00% <0.00%> (+0.24%)	⬆️
google/cloud/storage/internal/curl_handle.cc	73.03% <0.00%> (+0.56%)	⬆️
google/cloud/bigtable/internal/common_client.h	100.00% <0.00%> (+5.97%)	⬆️

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[dbolduc]

is "_subscrption_wit_" in the title intentional?

[coryan]

is "subscrption_wit" in the title intentional?

Thanks, fixed.

[coryan]

/cc: @anguillanneuf