Documentation: fix spelling in design-patterns

This fixes two spelling mistakes in the design pattern doc.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>