ref-manual: fix typo and move SYSROOT_DIRS example

1. Changed one letter (s/B/A), so the sentence is correct again.
2. Moved example from SYSROOT_DIRS_IGNORE into SYSROOT_DIRS section.

(From yocto-docs rev: 9fcd6d6ef4a87f69b8a00907051c1ece41e75a82)

Signed-off-by: Matthias Pritschet <matthias@pritschet.eu>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>