Wrong example of creating PbjGrpcClient

Hello,
please show actual example of using PbjGrpcClient because usage-guide is wrong.

```java
PbjGrpcClient grpcClient = PbjGrpcClient.builder()
.host("localhost")
.port(8080)
.build();

```
Builder method not exists.

Start with the usage-guide entry for PbjGrpcClient and compare its example with the current Java API, focusing on the builder call shown in the issue. Replace the invalid example with a working construction example and verify that the documented methods match the implementation.

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