fix link for reference semantics vignette (#7489)

Author: ben-schwen

man/assign.Rd

@@ -74,7 +74,7 @@ All of the following result in a friendly error (by design) :
 
 For additional resources, please read \href{../doc/datatable-faq.html}{\code{vignette("datatable-faq")}}. Also have a look at StackOverflow's \href{https://stackoverflow.com/questions/tagged/data.table/}{data.table tag}.
 
-\code{:=} in \code{j} can be combined with all types of \code{i} (such as binary search), and all types of \code{by}. This a one reason why \code{:=} has been implemented in \code{j}. Please see \href{../doc/datatable-reference-semantics}{\code{vignette("datatable-reference-semantics")}} and also \code{FAQ 2.16} for analogies to SQL.
+\code{:=} in \code{j} can be combined with all types of \code{i} (such as binary search), and all types of \code{by}. This a one reason why \code{:=} has been implemented in \code{j}. Please see \href{../doc/datatable-reference-semantics.html}{\code{vignette("datatable-reference-semantics")}} and also \code{FAQ 2.16} for analogies to SQL.
 
 When \code{LHS} is a factor column and \code{RHS} is a character vector with items missing from the factor levels, the new level(s) are automatically added (by reference, efficiently), unlike base methods.