docs: Fix Dub section being inside CMake section [skip ci]
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.
QuLogic/meson,pexip/meson,mesonbuild/meson,jpakkane/meson,QuLogic/meson,becm/meson,becm/meson,jpakkane/meson,jpakkane/meson,mesonbuild/meson,becm/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson,mesonbuild/meson,QuLogic/meson,mesonbuild/meson,QuLogic/meson,jpakkane/meson,jpakkane/meson,becm/meson,QuLogic/meson,pexip/meson,becm/meson,QuLogic/meson,becm/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,QuLogic/meson,becm/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,QuLogic/meson,mesonbuild/meson,becm/meson,mesonbuild/meson,mesonbuild/meson,becm/meson,jpakkane/meson,jpakkane/meson,becm/meson,pexip/meson,QuLogic/meson,pexip/meson