models: rename Checksums -> Checksum
models: rename Checksums -> Checksum

for uniformatiy with the other models
the table is still called "checksums" (for uniformity too)

sophiejjj/debsources,Debian/debsources,sophiejjj/debsources,nonas/debian-qa,vivekanand1101/debsources,sophiejjj/debsources,nonas/debian-qa,devoxel/debsources,oorestisime/debsources,zacchiro/debsources,sophiejjj/debsources,zacchiro/debsources,devoxel/debsources,clemux/debsources,clemux/debsources,vivekanand1101/debsources,clemux/debsources,Debian/debsources,zacchiro/debsources,Debian/debsources,matthieucan/debsources,zacchiro/debsources,oorestisime/debsources,matthieucan/debsources,matthieucan/debsources,matthieucan/debsources,vivekanand1101/debsources,devoxel/debsources,devoxel/debsources,Debian/debsources,vivekanand1101/debsources,zacchiro/debsources,oorestisime/debsources,Debian/debsources,clemux/debsources,sophiejjj/debsources,nonas/debian-qa,vivekanand1101/debsources,matthieucan/debsources,oorestisime/debsources,clemux/debsources,nonas/debian-qa,devoxel/debsources,oorestisime/debsources