[LTP] [PATCH v3] doc: correct the build steps for open_posix_testsuite

[Petr Vorel]

Hi Xiubo Li,

> './configure --with-open-posix-testsuite' is needed just before
> generating the Makefiles.

Thanks!

Reviewed-by: Petr Vorel <pvorel@suse.cz>

Kind regards,
Petr