[qty.dim.types] Fix derived_dimension example

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

docs/api_reference/src/quantities.tex

@@ -915,7 +915,7 @@
 to represent the dimension of a derived quantity\irefiev{112-01-10}.
 \begin{example}
 \begin{codeblock}
-constexpr auto acceleration = isq::dim_speed / isq::dim_time;
+constexpr auto dim_acceleration = isq::speed.dimension / isq::dim_time;
 static_assert(dealias(^decltype(acceleration)) ==
               ^derived_dimension<isq::dim_length, per<power<isq::dim_time, 2>>>);
 \end{codeblock}