Small improvement on maliput overview's picture

[francocipollone]

Summary

In the Maliput Overview section there is this image:

While it is ok, there could be improvements to better explain the difference between the lane, segment and junction entities.

Proposal

• Segments A, B, and C should be part of a junction.
• 1_0, 2_0, 3_0 shouldn't be part of the same intersection junction, they should be different junctions.

Also, demo maps like TShapeRoad would follow this pattern for the intersection.