Fix missing flatc in doc build (#483)

Summary:
I missed the step to install flatc in doc build workflow after https://github.com/pytorch/executorch/pull/465.  And this also fixes 2 minor leftover typos in `docs/source/index.rst` (from https://hud.pytorch.org/pytorch/executorch/commit/c3a6a6af054c346331fb6c76a3472571168bb50f)

Pull Request resolved: https://github.com/pytorch/executorch/pull/483

Reviewed By: cccclai

Differential Revision: D49614858

Pulled By: huydhn

fbshipit-source-id: 63395bb109f502f2c83b6e699f1e9e086e3aa19a