tools: fix warning in doc parsing

The description of "[start[, end]]" in the doc shows warning of
"invalid param" when parsing an optional parameter in the section.
This fixes insufficient trimming of right square brackets.

PR-URL: https://github.com/nodejs/node/pull/4537


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>