Fix typo in user-facing warning in fmelt.
Fix typo in user-facing warning in fmelt.

Rdatatable/data.table,vlulla/data.table,Rdatatable/data.table,Rdatatable/data.table,jangorecki/data.table,jangorecki/data.table,jangorecki/data.table,Rdatatable/data.table,vlulla/data.table,vlulla/data.table,jangorecki/data.table,vlulla/data.table