response_body/problem.md: update phrasing
response_body/problem.md: update phrasing

This should polish the phrasing a little. Nothing too major, but it might be helpful. Thanks!
koajs/kick-off-koa,koajs/kick-off-koa,satanas/kick-off-koa,deliciousinsights/kick-off-koa,StephanieBeaton/kick-off-koa