Minor changes in Section 4.1.3

Small changes to improve readability are proposed:

From "Passive structure elements are often information or data objects, but they can also represent physical objects." to "Passive structure elements may represent information or data objects in the IT world, and physical objects in the real world."

Add after Figure 11: "Passive structure elements are denoted using boxes with square corners and an icon in the upper-right corner, or by the icon on its own."