FIX: Broken links on "new-color-scheme-roles.md" page

The following four links to reference pages on the Flutter web site page https://docs.flutter.dev/release/breaking-changes/new-color-scheme-roles are broken:

- [Support fidelity variant for ColorScheme.fromSeed](flutter/flutter#144649)
- [Introduce tone-based surfaces and accent color add-ons - Part 1](flutter/flutter#142654)
- [Introduce tone-based surfaces and accent color add-ons - Part 2](flutter/flutter#138521)
- [Enhance ColorScheme.fromSeed with a new variant parameter](flutter/flutter#144805)

They are broken due to wrong syntax in the markdown file, used URLs are otherwise correct, but the used extended markdown link syntax is not, causing the links to be broken.

This PR fixes the error so the links work.

src/content/release/breaking-changes/new-color-scheme-roles.md

@@ -154,7 +154,7 @@ Relevant PRs:
 * [Enhance ColorScheme.fromSeed with a new variant parameter][]
 
 [Support tone-based surface and surface container ColorScheme roles]: {{site.repo.flutter}}/issues/115912
-[Support fidelity variant for ColorScheme.fromSeed]: {{site.repo.flutter}}/issues/[144649]
-[Introduce tone-based surfaces and accent color add-ons - Part 1]: {{site.repo.flutter}}/pull/[142654]
-[Introduce tone-based surfaces and accent color add-ons - Part 2]: {{site.repo.flutter}}/pull/[144273]
-[Enhance ColorScheme.fromSeed with a new variant parameter]: {{site.repo.flutter}}/pull/[144805]
+[Support fidelity variant for ColorScheme.fromSeed]: {{site.repo.flutter}}/issues/144649
+[Introduce tone-based surfaces and accent color add-ons - Part 1]: {{site.repo.flutter}}/pull/142654
+[Introduce tone-based surfaces and accent color add-ons - Part 2]: {{site.repo.flutter}}/pull/144273
+[Enhance ColorScheme.fromSeed with a new variant parameter]: {{site.repo.flutter}}/pull/144805