Correct Bucket#[]= documentation

thanks to @vmx

Change-Id: I3fcf7774719787e809922d42aecdbef11ce1e3d5
Reviewed-on: http://review.couchbase.org/25333
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>