[alg.reverse] remove duplicated Requires: element

Fixes NB JP 14 (C++17 DIS)

Author: jwakely

source/algorithms.tex

@@ -3201,7 +3201,8 @@
 \begin{itemdescr}
 \pnum
 \requires
-\tcode{*first} shall be swappable\iref{swappable.requirements}.
+\tcode{BidirectionalIterator} shall satisfy the requirements of
+\tcode{ValueSwappable}\iref{swappable.requirements}.
 
 \pnum
 \effects
@@ -3212,11 +3213,6 @@
 to all pairs of iterators
 \tcode{first + i, (last - i) - 1}.
 
-\pnum
-\requires
-\tcode{BidirectionalIterator} shall satisfy the requirements of
-\tcode{ValueSwappable}\iref{swappable.requirements}.
-
 \pnum
 \complexity
 Exactly