Fix typos and improve Gson Design Document formatting

[Marcono1234]

Purpose

Fixes some typos and grammar issues and tries to improve the Gson Design Document formatting.

Checklist

• New code follows the Google Java Style Guide
• If necessary, new public API validates arguments, for example rejects null
• New public API has Javadoc
  • Javadoc uses @since $next-version$
    ($next-version$ is a special placeholder which is automatically replaced during release)
• If necessary, new unit tests have been added
  • Assertions in unit tests use Truth, see existing tests
  • No JUnit 3 features are used (such as extending class TestCase)
  • If this pull request fixes a bug, a new test was added for a situation which failed previously and is now fixed
• mvn clean verify javadoc:jar passes without errors

[Marcono1234]

I removed some redundant trailing whitespace in this file. Unfortunately due to that the GitHub diff UI fails to properly show the changes.

When reviewing the changes in this file, please temporarily configure your Git client or the GitHub UI to ignore whitespace changes.

Or should I revert the whitespace changes?

[eamonnmcmanus]

This all looks fine. Thanks!