[alg.contains] Use codeblock for long return expression

Author: tkoeppe

source/algorithms.tex

@@ -4579,7 +4579,10 @@
 \begin{itemdescr}
 \pnum
 \returns
-\tcode{first2 == last2 || !ranges::search(first1, last1, first2, last2, pred, proj1, proj2).empty()}.
+\begin{codeblock}
+first2 == last2 || !ranges::search(first1, last1, first2, last2,
+                                   pred, proj1, proj2).empty()
+\end{codeblock}
 \end{itemdescr}
 
 \begin{itemdecl}