kbuild: fix typos (s/bin_shipped/bin.o_shipped/) in Documentation

The text always mentions ...bin.o_shipped, just the example makefiles
actually use ...bin_shipped. It was corrected in one place some time
ago, these ones seem to have been forgotten.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>