This Rotation Matrix rotates a vector around the x-axis by 90 degrees. Your vector initially points toward the z-axis and after the rotation it points in y-direction. I think this is counter-clock-wise as it's written in the article.

This Rotation Matrix rotates around the x-axis by 90 degrees. Your vector initially point toward the z-axis and after the rotation it point in y-direction. I think this is counter-clock-wise as it's written in the article.