Clarify language in error section

[Brad Jones]

Section 5.1 regarding error objects states,

"The remainder of the space is available for application defined errors."

I am working with the Drupal implementation <https://www.drupal.org/project/jsonrpc/issues/3283740> and updating the internal validation of error codes. I am wanting to communicate a client error (rate limiting) and that would fall outside of the scope of the predefined codes, and I _think_ I can pick any integer. However the above language is confusing as to the meaning of "the space." I think this means the set of all integers, but it is confusing as written.

Brad