Minor typo fix

Ss18/pr0

Fix typo (tho -> two)

Update README.md

Fixed typo

Fixed "remember" spelled as "rember" and fixed a comma wrongfuly placed.

Update README.md with SDL error issue

Fix typos and 404's

'xdd' should be 'xxd'

Fix typo.

change gcc src download link

In the lesson 13 makefile, os-image.bin can be shortened to $@

Fix comment for number of sectors to read

update READMEs

fixed typo