Summary:
Small pull request

 **motivation**
The doc 'Building React Native from source' is not up to date now.

While i try to build from the source my self, the gradle-download-task(2.0.0) will always fail to download boost zip with a failure of 403.

After i upgrade the gradle-download-task to 3.1.2, the download works well.

**Test plan (required)**
I tested to build the source myself, it works.

This is non-code update, it is document update, hope to help other people who get the same download failure.

**Code formatting**
Non-code update PR.
Closes https://github.com/facebook/react-native/pull/11298

Differential Revision: D4288387

fbshipit-source-id: 5e6feef1ea80f7e3feddcc7e4abf2b5cea82f08f