Fix typos

Fix a few trivial typos that I noticed when reading the PDF.

The first patch, fixing the spelling error "verison" -> "version" in a commit message, would of course also alter the SHA-1s of the commits in the example.  But it doesn't look like there is any intention that the reader could replicate the SHA-1s, so I don't think it is a problem that the SHA-1 shown could not actually result from a commit with the log message in its corrected form.