[stable][docs] Fix broken indentation in CHANGELOG.

Indentation and markdown wrapping in the CHANGELOG somehow broke on the latest commit in the Dart 1.7 section.

This CL rewrites the CHANGELOG entries to fix the formatting.

Change-Id: I5ae764c58abf7576e837ddff9f53ddcd5ccdb41b
Cherry-pick: This is not a cherry-pick, it is a correction to a mistake
Cherry-pick-request: N/A
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/324761
Reviewed-by: Jonas Termansen <[email protected]>
Reviewed-by: William Hesse <[email protected]>
Commit-Queue: Kevin Chisholm <[email protected]>

Author: itsjustkevin

CHANGELOG.md

@@ -8515,4 +8515,4 @@ documentation on the [Dart API site](http://api.dart.dev).
     compression.
 
 - `dart:isolate`: `Isolate.spawnUri` added the optional `packageRoot` argument,
-which controls how it resolves `package:` URIs.
+  which controls how it resolves `package:` URIs.