Fix links in contributing guidelines
Fix links in contributing guidelines

owncloud/gallery,patman15/galleryplus,IATkachenko/gallery,efueger/galleryplus,owncloud/gallery,desaintmartin/gallery,tahaalibra/gallery,interfasys/galleryplus,imjalpreet/gallery,desaintmartin/gallery,patman15/galleryplus,setnes/gallery,setnes/gallery,setnes/galleryplus,viraj96/gallery,tahaalibra/gallery,imjalpreet/gallery,desaintmartin/gallery,viraj96/gallery,owncloud/galleryplus,setnes/galleryplus,owncloud/gallery,patman15/galleryplus,setnes/galleryplus,owncloud/galleryplus,leftees/galleryplus,owncloud/galleryplus,interfasys/galleryplus,leftees/galleryplus,efueger/galleryplus,IATkachenko/gallery,IATkachenko/gallery,interfasys/galleryplus,imjalpreet/gallery,setnes/gallery,leftees/galleryplus,viraj96/gallery,tahaalibra/gallery