Fix typo in _flash_messages.scss
Fix typo in _flash_messages.scss

There is a missing s in „&.alert-success“.

Fixes #6797.

Amadren/diaspora,geraspora/diaspora,geraspora/diaspora,Muhannes/diaspora,jhass/diaspora,jhass/diaspora,SuperTux88/diaspora,Flaburgan/diaspora,Muhannes/diaspora,KentShikama/diaspora,Flaburgan/diaspora,spixi/diaspora,geraspora/diaspora,SuperTux88/diaspora,diaspora/diaspora,KentShikama/diaspora,spixi/diaspora,despora/diaspora,spixi/diaspora,diaspora/diaspora,jhass/diaspora,Amadren/diaspora,Amadren/diaspora,Flaburgan/diaspora,Muhannes/diaspora,diaspora/diaspora,KentShikama/diaspora,geraspora/diaspora,jhass/diaspora,despora/diaspora,SuperTux88/diaspora,Amadren/diaspora,Flaburgan/diaspora,spixi/diaspora,Muhannes/diaspora,SuperTux88/diaspora,despora/diaspora,diaspora/diaspora,despora/diaspora,KentShikama/diaspora