docs: run `meson setup` explicitly in build instructions

This silences the following warning given on newer version of meson:

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

doc/build.rst

@@ -118,7 +118,7 @@ Following example script will:
 
 .. code-block:: bash
 
-   $ meson build_dir --prefix=/tmp/kr --default-library=static
+   $ meson setup build_dir --prefix=/tmp/kr --default-library=static
    $ ninja -C build_dir
    $ ninja install -C build_dir
 
@@ -181,7 +181,7 @@ target ``doc`` must be called explicitly.
 
 .. code-block:: bash
 
-   $ meson build_dir -Ddoc=enabled
+   $ meson configure build_dir -Ddoc=enabled
    $ ninja -C build_dir doc
 
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