remove unnecessary `/` in reference url

Created by: Rplus

https://github.com/twbs/bootstrap/blob/v4.0.0-alpha.2/scss%2Fmixins%2F_screen-reader.scss#L3

current reference URL of @mixin sr-only is 404

http://a11yproject.com/posts/how-to-hide-content/

Just remove unnecessary / in the end. http://a11yproject.com/posts/how-to-hide-content