designprinciples.rst: Perform minor cleanups

- Remove some missed wiki markup, and escape a "\n" correctly.
- Use gender-neutral language to refer to the user, consistently.

Cc: Claudius Heine <ch@denx.de>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>