b2: reduce default --b2-upload-concurrency to 4 to reduce memory usage

In v1.63 memory usage in the b2 backend was limited to `--transfers` *
`--b2-chunk-size`

However in v1.64 this was raised to `--transfers` * `--b2-chunk-size`
* `--b2-upload-concurrency`.

The default value for this was accidently set quite high at 16 which
means by default rclone could use up to 6.4GB of memory!

The new default sets a more reasonable (but still high) max memory of 1.6GB.

backend/b2/b2.go

@@ -158,7 +158,7 @@ concurrently.
 Note that chunks are stored in memory and there may be up to
 "--transfers" * "--b2-upload-concurrency" chunks stored at once
 in memory.`,
-			Default:  16,
+			Default:  4,
 			Advanced: true,
 		}, {
 			Name: "disable_checksum",