doc: remove space between argument and build folder

When using `zephyr-app-command` the CMake arguments and values are not
having spaces between them, except the `-B build`.

Remove the space, so that `-B build` becomes `-Bbuild` and thus looks
similar to other `-<arg><value>` occurences.

Example before this commit:
> Use cmake to configure a Ninja-based buildsystem:
> cmake -B build -GNinja -DBOARD=reel_board samples/hello_world
>
> Now run ninja on the generated build system:
> ninja -C build

With this commit:
> Use cmake to configure a Ninja-based buildsystem:
> cmake -Bbuild -GNinja -DBOARD=reel_board samples/hello_world
>
> Now run ninja on the generated build system:
> ninja -Cbuild

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>