Fix Bigtable docs references to "instances". (#1848)
Fix Bigtable docs references to "instances". (#1848)

Remove confusion between creating a "Compute Instance" (which is a GCE VM
instance) vs. a Cloud Bigtable instance and cluster (which is what we need
here).

Updated the link to point to the correct page in the documentation, as Bigtable
also has documentation for creating GCE VM instances with the correct scopes to
access Bigtable instances, but that's not what we want here, and not what the
code sample is trying to accomplish.
googleapis/nodejs-bigtable,googleapis/nodejs-bigtable,googleapis/nodejs-bigtable