[FIX] make mapwriter download URL OS independent

failed on Windows due to the inserted two backslashes

Author: treee111

wahoomc/downloader.py

@@ -117,7 +117,7 @@ def download_tooling():
     check here for new mapwriter plugin version: https://github.com/mapsforge/mapsforge
     """
     map_writer_filename = 'mapsforge-map-writer-0.21.0-jar-with-dependencies.jar'
-    mapwriter_plugin_url = os.path.join('https://search.maven.org/remotecontent?filepath=org/mapsforge/mapsforge-map-writer/0.21.0', map_writer_filename)
+    mapwriter_plugin_url = 'https://search.maven.org/remotecontent?filepath=org/mapsforge/mapsforge-map-writer/0.21.0/' + map_writer_filename
 
     # Windows
     if platform.system() == "Windows":