Improve `diff` argument name
Improve `diff` argument name

From discussion in https://github.com/moment/moment/pull/2916/files/f2e45889e5d7522d7a5e8ffd607b783be320cb3c#r60102231

maggiepint/moment,julionc/moment,scriptrdotio/moment,samuelfullerthomas/moment,calebcauthon/moment,mj1856/moment,samuelfullerthomas/moment,gabrielmaldi/moment,whytobe/moment,moment/moment,moment/moment,ichernev/moment,PKRoma/moment,maggiepint/moment,ze-pequeno/moment,Oire/moment,xkxx/moment,gabrielmaldi/moment,mj1856/moment,OtkurBiz/moment,scriptrdotio/moment,ze-pequeno/moment,julionc/moment,PKRoma/moment,whytobe/moment,OtkurBiz/moment,samuelfullerthomas/moment,ichernev/moment,joelmheim/moment,julionc/moment,SkReD/moment,mj1856/moment,joelmheim/moment,ze-pequeno/moment,Oire/moment,monoblaine/moment,moment/moment,calebcauthon/moment,PKRoma/moment,monoblaine/moment,xkxx/moment,OtkurBiz/moment,scriptrdotio/moment,calebcauthon/moment,joelmheim/moment,Oire/moment,maggiepint/moment,xkxx/moment