Merge pull request #439 from kariya-mitsuru/fix-typo

[swappable.requirements] add comma

Author: jwakely

source/lib-intro.tex

@@ -1447,7 +1447,7 @@
 
 // Requires: lvalues of \tcode{T} shall be swappable.
 template <class T>
-void lv_swap(T& t1 T& t2) {
+void lv_swap(T& t1, T& t2) {
   using std::swap;
   swap(t1, t2);                                 // OK: uses swappable conditions for
 }                                               // lvalues of type \tcode{T}