[docs] Replace wget command in gRPC docs with curl

Summary:
Linear issue with full context here. The current gRPC docs make use of the wget command to install protobuf from GitHub, but wget is not pre-installed on macOS.

Replaced wget with curl, which is guaranteed to be installed since the brew install command above the wget command installs curl:

Test Plan: Re-ran all the commands before and after the wget command in the comm home directory with the new curl command, made sure outputs were the same as with the wget command.

Reviewers: ashoat, varun, atul

Reviewed By: ashoat, varun

Subscribers: palys-swm, Adrian, atul

Differential Revision: https://phab.comm.dev/D4258