[set.{union,intersection}] Add missing "of".

This was mistakenly omitted in the application of P3179R9.

Author: tkoeppe

source/algorithms.tex

@@ -10428,7 +10428,7 @@
   and \tcode{proj1} and \tcode{proj2} be \tcode{identity\{\}}
   for the overloads with no parameters by those names;
 \item
-  $M$ be \tcode{last1 - first1} plus the number elements in \range{first2}{last2}
+  $M$ be \tcode{last1 - first1} plus the number of elements in \range{first2}{last2}
   that are not present in \range{first1}{last1};
 \item
   \tcode{result_last} be \tcode{result + $M$}
@@ -10559,7 +10559,7 @@
   and \tcode{proj1} and \tcode{proj2} be \tcode{identity\{\}}
   for the overloads with no parameters by those names;
 \item
-  $M$ be the number elements in \range{first1}{last1}
+  $M$ be the number of elements in \range{first1}{last1}
   that are present in \range{first2}{last2};
 \item
   \tcode{result_last} be \tcode{result + $M$}