bigtable: fix a lint casing nit
bigtable: fix a lint casing nit

Replace
    "clusterId"
with
    "clusterID"

which is a solely Go1.11 failure noticed after
CL https://code-review.googlesource.com/c/gocloud/+/42850

and in the CI
https://source.cloud.google.com/results/invocations/67d96958-1f68-4874-b706-4723a2db157f/targets/cloud-go-libraries%2Fgoogle-cloud-go%2Fcontinuous%2Fgo111/log
due to a rigid lint rule for Go1.11.

Change-Id: Id8cbcebdaa5d03e1a3a86cdda4fac669fe681435
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/42873
Reviewed-by: Alex Hong <3311379d571f0e1dc66d44c9732cbfa024b0fda6@google.com>
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go