doc(pubsub): fix typos Pub/Sub vs. Pubsub

[coryan]

The name of the service is Cloud Pub/Sub, fixed it in a number of
places.

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This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 4f4b23b4a17fbf3c4f1cd0caa90c1b2c21ef6762

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[codecov]

Codecov Report

Merging #9034 (4f4b23b) into main (c4fc35d) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #9034      +/-   ##
==========================================
- Coverage   94.48%   94.48%   -0.01%     
==========================================
  Files        1421     1421              
  Lines      125403   125403              
==========================================
- Hits       118488   118486       -2     
- Misses       6915     6917       +2     

Impacted Files	Coverage Δ
google/cloud/pubsub/message.h	100.00% <ø> (ø)
google/cloud/pubsub/mocks/mock_ack_handler.h	100.00% <ø> (ø)
...le/cloud/internal/default_completion_queue_impl.cc	97.15% <0.00%> (-0.57%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.75% <0.00%> (-0.25%)	⬇️

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