[time.duration.cons] Fix duplicate definition in example (#6372)

Author: hewillk

source/time.tex

@@ -1313,7 +1313,7 @@
 \begin{example}
 \begin{codeblock}
 duration<int, milli> d(3);          // OK
-duration<int, milli> d(3.5);        // error
+duration<int, milli> d2(3.5);       // error
 \end{codeblock}
 \end{example}