Outdated link to GSHHS shapefile for 1_build_graph_from_shapefiles.py: add this update and clarification

[nitram9]

In the examples folder and the 1_build_graph_from_shapefiles.py file, change the description from line 27:31 with the following:

# In this example a visibility graph will be created from the GSHHS shorelines
# available at: http://www.soest.hawaii.edu/pwessel/gshhg/
# 1. Download the shorelines in the format of shapefiles and select the wanted resolution 
#    and level (in this example we use GSHHS (type), Crude (quality), L1 (region)). 
# 2. Copy all the related files (shx, shp, prj, dbf) from the following subfolder (GSHHS -> c) 
#    and only the *C1 relevant files to the folder that contains this script.