Inconsistent code formatting / minor fixes to vignettes

vignettes/datatable-reference-semantics.Rmd

@@ -68,7 +68,7 @@ DF$c <- 18:13               # (1) -- replace entire column
 DF$c[DF$ID == "b"] <- 15:13 # (2) -- subassign in column 'c'
 ```
 
-both (1) and (2) resulted in deep copy of the entire data.frame in versions of `R` versions `< 3.1`. [It copied more than once](https://stackoverflow.com/q/23898969/559784). To improve performance by avoiding these redundant copies, *data.table* utilised the [available but unused `:=` operator in R](https://stackoverflow.com/q/7033106/559784).
+both (1) and (2) resulted in deep copy of the entire data.frame in versions of `R` `< 3.1`. [It copied more than once](https://stackoverflow.com/q/23898969/559784). To improve performance by avoiding these redundant copies, *data.table* utilised the [available but unused `:=` operator in R](https://stackoverflow.com/q/7033106/559784).
 
 Great performance improvements were made in `R v3.1` as a result of which only a *shallow* copy is made for (1) and not *deep* copy. However, for (2) still, the entire column is *deep* copied even in `R v3.1+`. This means the more columns one subassigns to in the *same query*, the more *deep* copies R does.