docs: Added -GNinja to cmake configuration commands

[tmatsuo]

docs: Also fixed the directory name for clang example

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This change is 

[codecov]

Codecov Report

Base: 93.97% // Head: 93.97% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (002c9dd) compared to base (c409811).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10532      +/-   ##
==========================================
- Coverage   93.97%   93.97%   -0.01%     
==========================================
  Files        1630     1630              
  Lines      147027   147027              
==========================================
- Hits       138170   138167       -3     
- Misses       8857     8860       +3     

Impacted Files	Coverage Δ
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	97.05% <0.00%> (-2.95%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	88.29% <0.00%> (-0.34%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.82% <0.00%> (-0.34%)	⬇️
google/cloud/pubsub/samples/samples.cc	87.85% <0.00%> (-0.08%)	⬇️
google/cloud/internal/async_connection_ready.cc	93.61% <0.00%> (+4.25%)	⬆️

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[tmatsuo]

@coryan Thanks, PTAL