[DOCS][Tiny] Added a missing dash(-) in docs/configuration.md

The first line had only two dashes (--) instead of three(---). Because of this missing dash(-), 'jekyll build' command was not converting configuration.md to _site/configuration.html

Author: Taka Shinagawa <taka.epsilon@gmail.com>

Closes #6513 from mrt/docfix3 and squashes the following commits:

c470e2c [Taka Shinagawa] Added a missing dash(-) preventing jekyll from converting configuration.md to html format