graphclient is missing a BatchRequestRequestBuilder

[andrueastman]

Taking a look at the example at

msgraph-sdk-php/docs/Examples.md

Line 658 in 7fa86c0

### 2. Send the batch request using the `BatchRequestBuilder`

The graph client is missing the setup of the BatchRequestRequestBuilder.

The code of the graphclient should have an batch() function returning the BatchRequestRequestBuilder so that the example/sample is simplified to

$batchResponse = $graphServiceClient->batch()->postAsync($batchRequestContent)->wait();