docs: Fix Dub section being inside CMake section [skip ci]
It should be a `##` heading, not a `###` heading.
Also make heading label consistent with the others.
@@ -181,7 +181,7 @@ should only be used if the CMake files are not stored in the project itself.
Additional CMake parameters can be specified with the `cmake_args` property.
-### Some notes on Dub
+## Dub
Please understand that meson is only able to find dependencies that
exist in the local Dub repository. You need to manually fetch and
