deactivate broken links and add a comment

Author: mosc9575

src/bokeh_sampledata/airport_routes.py

@@ -8,7 +8,9 @@
 
 License: `ODbL 1.0`_
 
-Sourced from https://openflights.org/data.html on September 07, 2017.
+Sourced from `https:/openflights.org/data.html/` on September 07, 2017.
+This historic link is not active anymore, but https:/openflights.org/ is
+still available.
 
 This module contains two pandas Dataframes: ``airports`` and ``routes``.
 

src/bokeh_sampledata/airports.py

@@ -8,7 +8,8 @@
 
 License: `Public Domain`_
 
-Sourced from USGS service http://services.nationalmap.gov on October 15, 2015.
+Sourced from USGS service `http://services.nationalmap.gov` on October 15, 2015.
+In the meantime this service was deactivated and is not available anymore.
 
 This module contains one pandas Dataframe: ``data``.