fix region tag (#3238)

Fixes #issue

> It's a good idea to open an issue first for discussion.

- [ ] I have followed [Sample Format Guide](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/master/SAMPLE_FORMAT.md)
- [ ] `pom.xml` parent set to latest `shared-configuration`
- [ ] Appropriate changes to README are included in PR
- [ ] API's need to be enabled to test (tell us)
- [ ] Environment Variables need to be set (ask us to set them)
- [x] **Tests** pass:   `mvn clean verify` **required**
- [x] **Lint**  passes: `mvn -P lint checkstyle:check` **required**
- [ ] **Static Analysis**:  `mvn -P lint clean compile pmd:cpd-check spotbugs:check` **advisory only**
- [x] Please **merge** this PR for me once it is approved.

bigtable/snippets/src/main/java/com/example/bigtable/Reads.java

@@ -34,6 +34,7 @@ public class Reads {
 
   // Write your code here.
   // [START_EXCLUDE]
+  // [START bigtable_reads_row]
   public static void readRow() {
     // TODO(developer): Replace these variables before running the sample.
     String projectId = "my-project-id";