operator+ -> binary_op in reduce's note.

Fixes n3554/n3554#60

Author: jaredhoberock

algorithms.html

@@ -539,7 +539,7 @@ <h1>Reduce</h1>
 
         <cxx-note>
           The primary difference between <code>reduce</code> and <code>accumulate</code> is that the behavior
-          of <code>reduce</code> may be non-deterministic for non-associative or non-commutative <code>operator+</code>.
+          of <code>reduce</code> may be non-deterministic for non-associative or non-commutative <code>binary_op</code>.
         </cxx-note>
       </cxx-function>
     </cxx-section>