Update README_Cloud_Datastream_to_Spanner.md

Improve documentation for shardingContextFilePath variable

Author: aasthabharill

v2/datastream-to-spanner/README_Cloud_Datastream_to_Spanner.md

@@ -73,7 +73,7 @@ on [Metadata Annotations](https://github.com/GoogleCloudPlatform/DataflowTemplat
 * **transformationClassName**: Fully qualified class name having the custom transformation logic.  It is a mandatory field in case transformationJarPath is specified. Defaults to empty.
 * **transformationCustomParameters**: String containing any custom parameters to be passed to the custom transformation class. Defaults to empty.
 * **filteredEventsDirectory**: This is the file path to store the events filtered via custom transformation. Default is a directory under the Dataflow job's temp location. The default value is enough under most conditions.
-* **shardingContextFilePath**: Sharding context file path in cloud storage is used to populate the shard id in spanner database for each source shard.It is of the format Map<stream_name, Map<db_name, shard_id>>.
+* **shardingContextFilePath**: Sharding context file path in cloud storage is used to populate the shard id in spanner database for each source shard. It expects a JSON file with the format: {"StreamToDbAndShardMap": Map<stream_name, Map<db_name, shard_id>>}
 * **tableOverrides**: These are the table name overrides from source to spanner. They are written in thefollowing format: [{SourceTableName1, SpannerTableName1}, {SourceTableName2, SpannerTableName2}]This example shows mapping Singers table to Vocalists and Albums table to Records. For example, `[{Singers, Vocalists}, {Albums, Records}]`. Defaults to empty.
 * **columnOverrides**: These are the column name overrides from source to spanner. They are written in thefollowing format: [{SourceTableName1.SourceColumnName1, SourceTableName1.SpannerColumnName1}, {SourceTableName2.SourceColumnName1, SourceTableName2.SpannerColumnName1}]Note that the SourceTableName should remain the same in both the source and spanner pair. To override table names, use tableOverrides.The example shows mapping SingerName to TalentName and AlbumName to RecordName in Singers and Albums table respectively. For example, `[{Singers.SingerName, Singers.TalentName}, {Albums.AlbumName, Albums.RecordName}]`. Defaults to empty.
 * **schemaOverridesFilePath**: A file which specifies the table and the column name overrides from source to spanner. Defaults to empty.