grpc: update contributing.md

[purnesh42H]

RELEASE NOTES: None

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.16%. Comparing base (09166b6) to head (9ed2efa).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8318      +/-   ##
==========================================
- Coverage   82.17%   82.16%   -0.01%     
==========================================
  Files         419      419              
  Lines       41992    41992              
==========================================
- Hits        34505    34504       -1     
+ Misses       6021     6020       -1     
- Partials     1466     1468       +2     

see 21 files with indirect coverage changes

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