clean up React Redux tutorial

[aqandrew]

• fix internal link to connect's definition
• fix typo in Setting up Redux Actions
• remove Link import in first Post.js snippet, since this tutorial doesn't cover creating a SinglePostPage
• move highlighted { Post } import to the PostsPage.js snippet that uses it
• fix typo in Redux Toolkit section
• add missing if before boolean in last paragraph

[taniarascia]

Thanks! The typo checkers never seem to get the typos that are still words 😅