doc: Fix formatting for child_process docs.

Checklist

• documentation is changed or added
• the commit message follows commit guidelines

Affected core subsystem(s)

doc

Description of change

Ref: https://github.com/nodejs/node/issues/6911

Remove unnecessary backslash in exec() option maxBuffer. Adjust CSS so that links formatted as code display a grey background.

Note that the CSS changes do affect DOM elements outside of those explicitly listed, API documents outside of child_process.html. For example, in this screen capture, you can see that TOC items with code formatting also get the grey background treatment.