fix(spanner): tweak the tag name of a FGAC sample

[devbww]

The sample specification document was ambiguous about whether one of the FGAC tag names should be plural or not. The one we chose lost the toss.

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This change is 

[google-cloud-cpp-bot]

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For commit: b169f6a4656987facda9e51146abf626eefee0fd

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[codecov]

Codecov Report

Base: 93.84% // Head: 93.84% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (b169f6a) compared to base (5046d76).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10266      +/-   ##
==========================================
- Coverage   93.84%   93.84%   -0.01%     
==========================================
  Files        1554     1554              
  Lines      142815   142815              
==========================================
- Hits       134029   134028       -1     
- Misses       8786     8787       +1     

Impacted Files	Coverage Δ
google/cloud/spanner/samples/samples.cc	62.24% <0.00%> (ø)
...cloud/pubsub/internal/subscription_session_test.cc	98.15% <0.00%> (-0.51%)	⬇️
google/cloud/pubsub/samples/samples.cc	90.77% <0.00%> (+0.07%)	⬆️
...ud/spanner/integration_tests/client_stress_test.cc	84.86% <0.00%> (+0.65%)	⬆️

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[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• google/cloud/spanner/samples/samples.cc:810, tag spanner_add_and_drop_database_role

You are about to delete 1 region tag.

• google/cloud/spanner/samples/samples.cc:810, tag spanner_add_and_drop_database_roles

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