[flat.set.modifiers] (first, last) should be rg (#6246)

Restore consistency with [flat.map.modifiers]/13, which already uses `ranges::distance(rg)`.

source/containers.tex

@@ -16955,7 +16955,7 @@
 \pnum
 \complexity
 $N$ + $M \log M$, where $N$ is \tcode{size()} before the operation and $M$
-is \tcode{distance(first, last)}.
+is \tcode{ranges::distance(rg)}.
 
 \pnum
 \remarks