docs/manual: fix minor spelling/grammar mistakes

This commit fixes a few minor spelling or grammar mistakes since the recent
additions to the manual (commits 0b100de2cf36d81910ab53978b8a379214a683ea to
7cbb476661b0cfa213d8885f3fe5d58823e84286).

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Merge remote-tracking branch 'mirror/master'