[docs] Replace make install command in gRPC docs with sudo make install

Summary: Linear task with full context here. The make install command results in a "Permission denied" error. When re-running the command with sudo, the command works.

Test Plan: Command works now.

Reviewers: ashoat, atul, varun

Reviewed By: ashoat, atul

Subscribers: palys-swm, Adrian, atul

Differential Revision: https://phab.comm.dev/D4259