[ci skip] Fix list item rendered display of 4.2 release notes.

I changed this to 4 spaces in #16103 to display correctly.

The reason: [List items may consist of multiple paragraphs. Each subsequent paragraph in a list item must be indented by either 4 spaces or one tab](https://github.com/rails/rails/pull/16102#issuecomment-48441279).

cc @chancancode