Fix typos in Branches chapter

My plans for this chapter:
* This change fixes only typos in the chapter on branching.
* I intend to follow up with another in due course where I have re-worded text in the chapter.
* Eventually my intention is to follow up with a lot of new content with details of how to make use of the various options in Git Extensions for branching. In particular I would like to cover advanced options when merging and rebasing, such as octopus merging, interactive rebasing, and rebasing a specific range. I make use of these features often in my job.