docs(bigtable): clean up CreateTable sample

[dbolduc]

The bigtable_hw_create_table region cares about making a CreateTable rpc, not creating a Table client.

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This change is 

[codecov]

Codecov Report

Base: 93.66% // Head: 93.65% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (0232bed) compared to base (fb18335).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10844      +/-   ##
==========================================
- Coverage   93.66%   93.65%   -0.01%     
==========================================
  Files        1715     1715              
  Lines      154214   154214              
==========================================
- Hits       144438   144436       -2     
- Misses       9776     9778       +2     

Impacted Files	Coverage Δ
...le/cloud/bigtable/examples/bigtable_hello_world.cc	90.90% <100.00%> (ø)
google/cloud/internal/timer_queue.cc	96.10% <0.00%> (-1.30%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.82% <0.00%> (-0.34%)	⬇️
google/cloud/pubsub/samples/samples.cc	86.94% <0.00%> (-0.15%)	⬇️
google/cloud/completion_queue_test.cc	97.32% <0.00%> (+0.19%)	⬆️
...le/cloud/storage/internal/curl_download_request.cc	88.96% <0.00%> (+0.33%)	⬆️
...ud/spanner/integration_tests/client_stress_test.cc	84.86% <0.00%> (+0.65%)	⬆️

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