Added reference to issue to NEWS.md

Author: AntonNM

NEWS.md

@@ -133,7 +133,7 @@ rowwiseDT(
 
 19. An integer overflow in `fread()` with lines longer than `2^(31/2)` bytes is prevented, [#6729](https://github.com/Rdatatable/data.table/issues/6729). The typical impact was no worse than a wrong initial allocation size, corrected later. Thanks to @TaikiSan21 for the report and @aitap for the fix.
 
-20. By reference assignments (':=') with functions that modified the data.table by reference e.g. (`foo=function(DT){modify(DT);return(1L)}`, `DT[,a:=foo(DT)]`) returned a mallformed data.table due to the modification of the targeted named column index ("a") during the j expression evaluation. Thanks @AntonNM for the report and fix.
+20. By reference assignments (':=') with functions that modified the data.table by reference e.g. (`foo=function(DT){modify(DT);return(1L)}`, `DT[,a:=foo(DT)]`) returned a mallformed data.table due to the modification of the targeted named column index ("a") during the j expression evaluation [#6768](https://github.com/Rdatatable/data.table/issues/6768). Thanks @AntonNM for the report and fix.
 
 ## NOTES