Cloud Spanner: fix stale read comment.

Author: jsimonweb

spanner/src/read_stale_data.php

@@ -28,7 +28,8 @@
 use Google\Cloud\Spanner\SpannerClient;
 
 /**
- * Reads sample data from the database.  The data is exactly 10 seconds stale.
+ * Reads sample data from the database.  The data is exactly 15 seconds stale.
+ * Guarantees that all writes committed more than 15 seconds ago are visible.
  * Example:
  * ```
  * read_stale_data