[mp.units.core.syn] Fix derived_quantity_spec's template parameter

Co-authored-by: Mateusz Pusz <[email protected]>

docs/api_reference/src/quantities.tex

@@ -133,7 +133,7 @@
   requires @\seebelownc@
 struct quantity_spec<QS, Eq, Args...>;
 
-template<typename Expr>
+template<typename... Expr>
 struct derived_quantity_spec;
 
 struct dimensionless;