Update README to reflect changes in grpcio usage instructions

Author: mokucher

README.md

@@ -538,13 +538,13 @@ protoc \
 
 ### Using grpcio library instead of grpclib
 
-In order to use the `grpcio` library instead of `grpclib`, you can use the `--custom_opt=USE_GRPCIO`
+In order to use the `grpcio` library instead of `grpclib`, you can use the `--python_betterproto_opt=USE_GRPCIO`
 option when running the `protoc` command. 
 This will generate stubs compatible with the `grpcio` library.
 
 Example:
 ```sh
-protoc --custom_opt=USE_GRPCIO -I . --custom_out=generated --plugin=protoc-gen-custom=src/betterproto/plugin/main.py demo.proto
+protoc -I . --python_betterproto_out=.  --python_betterproto_opt=USE_GRPCIO demo.proto
 ```
 ### TODO