README: Move to "meson setup [options]"

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

Seen a few other repos do this like:
openbmc/phosphor-hwmon@46a29b7

Tested: The following work:
meson setup builddir
ninja -C builddir

Change-Id: I3176fbf0df064c17473f5fc6ad723ca03ed0d04f
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>

README.md

@@ -55,7 +55,7 @@ options are documented in `meson_options.txt`
 ## Compile bmcweb with default options
 
 ```ascii
-meson builddir
+meson setup builddir
 ninja -C builddir
 ```