Fix typos
Fix a couple of typos in the handbook.
This branch also removes the trailing spaces at the end of the lines in the modified files. I quickly had a look in the docs and some merge requests but didn't see any specification on whether it's allowed to have trailing spaces or not. I have just seen that @fatihacet has created a MR for this here: gitlab-org/gitlab-foss#20180 (moved), so I went ahead with the default settings of my editor.