[alg.reverse] remove duplicated Requires: element

Fixes NB JP 14 (C++17 DIS)

Author: jwakely

source/algorithms.tex

@@ -3188,7 +3188,8 @@
 \begin{itemdescr}
 \pnum
 \requires
-\tcode{*first} shall be swappable~(\ref{swappable.requirements}).
+\tcode{BidirectionalIterator} shall satisfy the requirements of
+\tcode{ValueSwappable}~(\ref{swappable.requirements}).
 
 \pnum
 \effects
@@ -3199,11 +3200,6 @@
 to all pairs of iterators
 \tcode{first + i, (last - i) - 1}.
 
-\pnum
-\requires
-\tcode{BidirectionalIterator} shall satisfy the requirements of
-\tcode{ValueSwappable}~(\ref{swappable.requirements}).
-
 \pnum
 \complexity
 Exactly