DOC: book 2 chapter 4 Segmentation edits

 TODO: Fix LevelSet Segmentation image size

more edits

Change-Id: I7dda75f12e18791ad93e5129d9e7cfe8c424dbc0
Merge topic 'book-2-chapter-4-edits'

d1bc3168 DOC: book 2 chapter 4 Segmentation edits