specify namespace for :=

Author: ben-schwen

R/data.table.R

@@ -2884,7 +2884,7 @@ address = function(x) .Call(Caddress, eval(substitute(x), parent.frame()))
 
 ":=" = function(...) {
   # this error is detected when eval'ing isub and replaced with a more helpful one when using := in i due to forgetting a comma, #4227
-  stopf('Check that is.data.table(DT) == TRUE. Otherwise, `:=` is defined for use in j, once only and in particular ways. See help(":="). A common reason for this error is allocating a new column in `j` and using `<-` instead of `:=`; e.g., `DT[, new_col <- 1]` should be `DT[, new_col := 1]`. Another is using `:=` in a multi-statement `{...}` block; please use `:=` as the only statement in `j`.', class="dt_invalid_let_error")
+  stopf('Check that is.data.table(DT) == TRUE. Otherwise, `:=` is defined for use in j, once only and in particular ways. See help(":=", "data.table"). A common reason for this error is allocating a new column in `j` and using `<-` instead of `:=`; e.g., `DT[, new_col <- 1]` should be `DT[, new_col := 1]`. Another is using `:=` in a multi-statement `{...}` block; please use `:=` as the only statement in `j`.', class="dt_invalid_let_error")
 }
 
 # TODO(#6197): Export these.