[qty.dim.types] Fix base of dimensionless

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

docs/api_reference/src/quantities.tex

@@ -1192,7 +1192,7 @@
 \begin{codeblock}
 namespace mp_units {
 
-inline constexpr struct @\libglobal{dimensionless}@ final : derived_quantity_spec<> {
+inline constexpr struct @\libglobal{dimensionless}@ final : quantity_spec<derived_quantity_spec<>> {
 } @\libglobal{dimensionless}@;
 
 }