Small typo in second section of Chapter 10 (plus Summary and repo Readme)

[jdsandifer]

The second section of Chapter 10 reads:

Using React's context (prior v. 16.3)

Since prior is an adjective (or noun), one of these would probably read better for the part in parentheses:

1. prior to v. 16.3
2. below v. 16.3 (this would align with "v. 16.3 and above" in the next section)
3. before v. 16.3

Whichever change is chosen, the same change would be needed in Chapter 10 itself (book/chapter-10/README.md), the Summary (book/SUMMARY.md), and the Content section of the repo readme (README.md).