Improve docs

[Breefield]

In https://square.github.io/wire/wire_grpc/#wire-grpc-server, the docs indicate

classpath("com.squareup.wiregrpcserver:server-generator:<new-repo-version>")

I think they would be more clear if they said:

buildscript {
    dependencies {
        classpath("com.squareup.wiregrpcserver:server-generator:<new-repo-version>")
    }
}