docs(storage-drivers/gcs): correct the documented chunksize default

The GCS storage driver parameters table documents `chunksize` as
defaulting to 5242880 (5 MiB). The driver does not use that value:
`registry/storage/driver/gcs/gcs.go` defines

	defaultChunkSize = 16 * 1024 * 1024

and applies it when the `chunksize` parameter is absent, so the documented
default understates the actual chunk size by a factor of three and a third.

Correct the table to 16777216 (16 MiB).

Signed-off-by: yuwk <1729065730@qq.com>

docs/content/storage-drivers/gcs.md

@@ -13,7 +13,7 @@ An implementation of the `storagedriver.StorageDriver` interface which uses Goog
 | `bucket`  | yes | The name of your Google Cloud Storage bucket where you wish to store objects (needs to already be created prior to driver initialization). |
 | `keyfile`  | no | A private service account key file in JSON format used for [Service Account Authentication](https://cloud.google.com/storage/docs/authentication#service_accounts). |
 | `rootdirectory`  | no | The root directory tree in which all registry files are stored. Defaults to the empty string (bucket root). If a prefix is used, the path `bucketname/<prefix>` has to be pre-created before starting the registry. The prefix is applied to all Google Cloud Storage keys to allow you to segment data in your bucket if necessary.|
-| `chunksize`  | no (default 5242880) | This is the chunk size used for uploading large blobs, must be a multiple of 256*1024. |
+| `chunksize`  | no (default 16777216) | This is the chunk size used for uploading large blobs, must be a multiple of 256*1024. |
 
 {{< hint type=note >}}
 Instead of a key file you can use [Google Application Default Credentials](https://developers.google.com/identity/protocols/application-default-credentials).