URL for biothings hub config example needs to be fixed in documentation

[kevinxin90]

In http://docs.biothings.io/en/latest/doc/multiple_sources_tutorial.html

Before starting to implement our hub, we first need to define a configuration file. A config_common.py https://github.com/biothings/biothings.species/blob/master/src/config_common.py

The link provided here is no longer valid. Need to be updated.