fix(grpc): correct prefix for project resources

[coryan]

This was breaking some of my integration tests for the C++ client library, but only for buckets created with gRPC. Sigh.

[codecov]

Codecov Report

Merging #280 (7ea5006) into main (b6f22e0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #280   +/-   ##
=======================================
  Coverage   97.62%   97.62%           
=======================================
  Files          48       48           
  Lines        7629     7632    +3     
=======================================
+ Hits         7448     7451    +3     
  Misses        181      181           

Flag	Coverage Δ
unittests	97.62% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
gcs/bucket.py	98.62% <100.00%> (ø)
tests/test_bucket_grpc.py	96.29% <100.00%> (+0.14%)	⬆️
tests/test_grpc_server.py	99.84% <100.00%> (+<0.01%)	⬆️

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