Fix image links in masks.rst
Fix image links in masks.rst

youngpm/rasterio,kapadia/rasterio,clembou/rasterio,brendan-ward/rasterio,kapadia/rasterio,brendan-ward/rasterio,perrygeo/rasterio,johanvdw/rasterio,youngpm/rasterio,njwilson23/rasterio,youngpm/rasterio,perrygeo/rasterio,perrygeo/rasterio,johanvdw/rasterio,clembou/rasterio,kapadia/rasterio,njwilson23/rasterio,clembou/rasterio,johanvdw/rasterio,njwilson23/rasterio,brendan-ward/rasterio