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.

packages/bigtable/src/index.js

@@ -71,10 +71,10 @@ var Instance = require('./instance.js');
  * //-
  *
  * //-
- * // <h3>Creating a Compute Instance</h3>
+ * // <h3>Creating a Bigtable Instance and Cluster</h3>
  * //
- * // Before you create your table, you first need to create a Compute Instance
- * // for the table to be served from.
+ * // Before you create your table, you first need to create a Bigtable Instance
+ * // and cluster for the table to be served from.
  * //-
  * var callback = function(err, instance, operation) {
  *   operation
@@ -99,7 +99,7 @@ var Instance = require('./instance.js');
  * //-
  * // This can also be done from either the Google Cloud Platform Console or the
  * // `gcloud` cli tool. Please refer to the
- * // <a href="https://cloud.google.com/bigtable/docs/creating-compute-instance">
+ * // <a href="https://cloud.google.com/bigtable/docs/creating-instance">
  * // official Bigtable documentation</a> for more information.
  * //-
  *