Modify districts in liberia.json

Modify districts in liberia.json based on document https://docs.google.com/a/ehealthnigeria.org/document/d/1Y1mf79yo4AKgXK_D3vhrKBfy42t2PNY4dFZVI6ay1QE/edit#
Merge pull request #14 from carolinagc/liberia

Modify districts in liberia.json