[is.sorted] Add missing "return" and semi-colon

This was lost when changing "Returns:" to "Effects:" for P0896R4. The
paper included this change, but it was lost when applying it.

Author: jwakely

source/algorithms.tex

@@ -6200,7 +6200,7 @@
 \effects
 Equivalent to:
 \begin{codeblock}
-is_sorted_until(std::forward<ExecutionPolicy>(exec), first, last, comp) == last
+return is_sorted_until(std::forward<ExecutionPolicy>(exec), first, last, comp) == last;
 \end{codeblock}
 \end{itemdescr}