docs: fix broken link for TESTING.md (#2126)

Fixes b/290932427

TESTING.md

@@ -0,0 +1,25 @@
+## Testing code that uses Google Cloud Storage
+
+`RemoteStorageHelper` contains convenience methods to make setting up and cleaning up the test buckets easier. To use this class:
+
+1. Create a test Google Cloud project.
+
+2. Download a JSON service account credentials file from the Google Developer's Console.  See more about this on the [Google Cloud Platform Storage Authentication page][cloud-platform-storage-authentication].
+
+3. Create a `RemoteStorageHelper` object using your project ID and JSON key.
+   Here is an example that uses the `RemoteStorageHelper` to create a bucket.
+   ```java
+   RemoteStorageHelper helper =
+     RemoteStorageHelper.create(PROJECT_ID, new FileInputStream("/path/to/my/JSON/key.json"));
+   Storage storage = helper.getOptions().getService();
+   String bucket = RemoteStorageHelper.generateBucketName();
+   storage.create(BucketInfo.of(bucket));
+   ```
+
+4. Run your tests.
+
+5. Clean up the test project by using `forceDelete` to clear any buckets used.
+   Here is an example that clears the bucket created in Step 3 with a timeout of 5 seconds.
+   ```java
+   RemoteStorageHelper.forceDelete(storage, bucket, 5, TimeUnit.SECONDS);
+   ```

google-cloud-storage/src/main/java/com/google/cloud/storage/testing/package-info.java

@@ -34,8 +34,7 @@
  * RemoteStorageHelper.forceDelete(storage, bucket, 5, TimeUnit.SECONDS);
  * }</pre>
  *
- * @see <a
- *     href="https://github.com/googleapis/google-cloud-java/blob/master/TESTING.md#testing-code-that-uses-storage">
- *     Google Cloud Java tools for testing</a>
+ * @see <a href="https://github.com/googleapis/java-storage/blob/main/TESTING.md">Google Cloud
+ *     Storage testing</a>
  */
 package com.google.cloud.storage.testing;