doc: west: build: fix info on --pristine default

In commit commit c19c6fb438b98e54d38eeff4a606d6d9a3d801c8
("Revert "scripts: west build: default build.pristine to auto""),
we set the default --pristine value back to 'never', but the
documentation never got updated. Fix it.

Signed-off-by: Martí Bolívar <mbolivar@amperecomputing.com>
1 file changed