There are some typos in boot image header and riscv boot documentation.

Fix the typos.
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Link: https://lore.kernel.org/r/20191009010637.9955-1-atish.patra@wdc.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>