Update SpannerIO.java - comment documentation (#38492)

Spanner supports 80,000 mutations per transacton

https://docs.cloud.google.com/spanner/quotas#limits_for_creating_reading_updating_and_deleting_data:~:text=Mutations%20per%20commit

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: googledrew

sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/spanner/SpannerIO.java

@@ -311,10 +311,10 @@
  *
  * <p>Note that the <a
  * href="https://cloud.google.com/spanner/quotas#limits_for_creating_reading_updating_and_deleting_data">maximum
- * size of a single transaction</a> is 20,000 mutated cells - including cells in indexes. If you
- * have a large number of indexes and are getting exceptions with message: <tt>INVALID_ARGUMENT: The
- * transaction contains too many mutations</tt> you will need to specify a smaller number of {@code
- * MaxNumMutations}.
+ * number of mutations in a single transaction</a> is 80,000 mutations - including mutations in
+ * indexes. If you have a large number of indexes and are getting exceptions with message:
+ * <tt>INVALID_ARGUMENT: The transaction contains too many mutations</tt> you will need to specify a
+ * smaller number of {@code MaxNumMutations}.
  *
  * <p>The batches written are obtained from by grouping enough {@link Mutation Mutations} from the
  * Bundle provided by Beam to form several batches. This group of {@link Mutation Mutations} is then