docs: Fix deprecated use of implicit meson "setup" command

docs: Fix deprecated use of implicit meson "setup" command

Use the explicit meson "setup" command instead of the deprecated
implicit invocation. The implied setup usage generates a warning with
meson version 1.0.1:

WARNING: Running the setup command as `meson [options]` instead of
`meson setup [options]` is ambiguous and deprecated.

Additionally the implicit command is likely to be confusing to those
unfamiliar with Meson - the `build` argument in `meson build` resembles
an action rather than an arbitrary path.

Signed-off-by: Tim Small <tim@seoss.co.uk>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>