bootstd: Fix a handful of doc typos in bootmeth

Fix some trivial typos found by browsing the code.
Done with flyspell.

Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Guillaume La <Roque&lt;glaroque@baylibre.com>
Reviewed-by: Julien Masson <jmasson@baylibre.com>