docs: use 'latest' stats package in samples to prevent build failures

The list of available statistics packages will change over time, meaning that the
samples need to use 'latest' to prevent build failures when a package has been
cleaned up by the backend.

Fixes #1273

samples/snippets/src/main/java/com/example/spanner/SpannerSample.java

@@ -1510,7 +1510,9 @@ static void clientWithQueryOptions(DatabaseId db) {
                 db, QueryOptions
                     .newBuilder()
                     .setOptimizerVersion("1")
-                    .setOptimizerStatisticsPackage("auto_20191128_14_47_22UTC")
+                    // The list of available statistics packages can be found by querying the
+                    // "INFORMATION_SCHEMA.SPANNER_STATISTICS" table.
+                    .setOptimizerStatisticsPackage("latest")
                     .build())
             .build();
     Spanner spanner = options.getService();
@@ -1538,6 +1540,8 @@ static void queryWithQueryOptions(DatabaseClient dbClient) {
                     .withQueryOptions(QueryOptions
                         .newBuilder()
                         .setOptimizerVersion("1")
+                        // The list of available statistics packages can be found by querying the
+                        // "INFORMATION_SCHEMA.SPANNER_STATISTICS" table.
                         .setOptimizerStatisticsPackage("latest")
                         .build())
                     .build())) {