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: nodejs#4537
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

tools/doc/json.js

@@ -283,7 +283,7 @@ function parseSignature(text, sig) {
     // [foo] -> optional
     if (p.charAt(p.length - 1) === ']') {
       optional = true;
-      p = p.substr(0, p.length - 1);
+      p = p.replace(/\]/g, '');
       p = p.trim();
     }
     var eq = p.indexOf('=');