README: specify go language in codeblock
README: specify go language in codeblock

Change-Id: Id7ad1d634a24ff5b1f7a38ad62253bc36541dbe0
Reviewed-on: https://code-review.googlesource.com/16230
Reviewed-by: Ross Light <dfccc06f14414bff5a59be7fc90abf4404b47fc8@google.com>

googleapis/google-api-go-client,googleapis/google-api-go-client,google/google-api-go-client,google/google-api-go-client