README.md, proto: remove usage of group from example

[wellsjo]

From Google's documentation:
https://developers.google.com/protocol-buffers/docs/proto#groups

Groups
Note that this feature is deprecated and should not be used when creating new message types – use nested message types instead.

[dsnet]

Where are you seeing groups being advertised?

[wellsjo]

In the README, right after

Consider file test.proto, containing

There is a code example that uses a group

[dsnet]

Ah. Got it. I was searching for "groups" instead and didn't see that.

[tmcrain]

I'll take a look!