doc: dfu: fix a handful of typos

Reword some sentences, add missing periods and fix various typos in the
dfu documentation.

This was originally contributed on [1]

[1] https://github.com/Maalobs/u-boot/commit/9a21ed8ba722d904b68945f40ca6697b4b1cc4d4



Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>