[PR] [DOCS] website: fix duplicate `declarations` path
[PR] [DOCS] website: fix duplicate `declarations` path

Summary:
Remove duplicate `declarations` nav-item on the [flowconfig page
![duplicate-entry](https://user-images.githubusercontent.com/14677379/67149672-bf5ca400-f2b6-11e9-8252-9cbef9c49ce1.png)
](https://flow.org/en/docs/config).
Pull Request resolved: https://github.com/facebook/flow/pull/8146

Differential Revision: D18117846

Pulled By: nmote

fbshipit-source-id: 87196d4293fe9acf866609a3b2a26f33b0aadc1d

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow