Fix for Issue 186 - Replace ref to `table-fields` in README with `entity-field`

The function was replaced by `entity-fields` in commit d75d1e1f70a14940809e1a6722e5879d2e7bbb96
Merge pull request #187 from danmidwood/master

Fix for Issue 186 - Replace ref to `table-fields` in README with `entity-fields`