doc: improve child_process `maxBuffer` text

`maxBuffer` information in child_process.md used atypical formatting.
This uses a single consistent style for all instances of `maxBuffer`
information.

PR-URL: https://github.com/nodejs/node/pull/11791


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>