doc: *.md formatting fixes in the benchmark dir

* Add language specification for the txt code blocks.
* Move the definitions to the bottom.

Ref: https://github.com/nodejs/node/pull/7727

PR-URL: https://github.com/nodejs/node/pull/7727


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>