fixed bug in an assertion message
fixed bug in an assertion message

slai11/youtube-8m,forwchen/yt8m,slai11/youtube-8m,hxkk/u8m_test,KangHsi/youtube-8m,coded5282/youtube-8m,slai11/youtube-8m,slai11/youtube-8m,forwchen/yt8m,lidalei/youtube-8m,KangHsi/youtube-8m,mwoodson1/youtube-8m-competition,google/youtube-8m