Merge pull request #1403 from cloudsufi/patch/update-doc-json-supported

[PLUGIN-1563] Update Json supported as string in documentation

Author: itsankit-google

docs/BigQueryTable-batchsource.md

@@ -105,4 +105,4 @@ corresponding CDAP data type for each BigQuery type.
 | struct        | record                             |
 | time          | time (microseconds)                |
 | timestamp     | timestamp (microseconds)           |
-| json          | unsupported                        |
+| json          | string                             |