correct fragLoadingTimeout doc according to config

[shacharz]

This PR will...

correct fragLoadingTimeout doc according to config

Why is this Pull Request needed?

config is actually 20s -

hls.js/src/config.js

Line 57 in 5820d29

fragLoadingTimeOut: 20000, // used by fragment-loader

Are there any points in the code the reviewer needs to double check?

Resolves issues:

Checklist

• changes have been done against master branch, and PR does not conflict
• no commits have been done in dist folder (we will take care of updating it)
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md

[johnBartos]

Thanks 👍