Correct typo (closes #705)

siunitx.tex

@@ -475,7 +475,7 @@ \subsection{Units}
     \cs{qtyproduct}\oarg{options}\marg{numbers}\marg{unit}
   \end{syntax}
 \end{function}
-Runs of products of of numbers with units can be handled using the
+Runs of products of numbers with units can be handled using the
 \cs{qtyproduct} function. The behaviour of this function is similar to
 \cs{numproduct}, but with the addition of a unit to each number.
 \begin{LaTeXdemo}