[alg.heap.operations] Use \bigoh, not \mathcal.

Author: jensmaurer

source/algorithms.tex

@@ -4666,7 +4666,7 @@
 or a new element added by
 \tcode{push_heap()},
 in
-$\mathcal{O}(\log N)$
+\bigoh{\log N}
 time.
 \end{itemize}