cleanup: fix usage string

[dbolduc]

We missed this extra argument refactoring storage benchmarks in #2671

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #7067 (6089ade) into main (d441eaf) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #7067      +/-   ##
==========================================
- Coverage   94.48%   94.48%   -0.01%     
==========================================
  Files        1304     1304              
  Lines      112377   112377              
==========================================
- Hits       106177   106174       -3     
- Misses       6200     6203       +3     

Impacted Files	Coverage Δ
google/cloud/testing_util/command_line_parsing.cc	98.57% <100.00%> (ø)
google/cloud/grpc_error_delegate.cc	95.83% <0.00%> (-4.17%)	⬇️
...bigtable/examples/bigtable_hello_instance_admin.cc	81.31% <0.00%> (-2.20%)	⬇️
google/cloud/pubsub/subscriber_connection_test.cc	97.20% <0.00%> (-0.70%)	⬇️
...sub/internal/batching_publisher_connection_test.cc	97.80% <0.00%> (+0.20%)	⬆️
...cloud/pubsub/internal/subscription_session_test.cc	98.02% <0.00%> (+0.24%)	⬆️

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