[docs] Fix link to ECS docs (#2563)

Fixes a link to the ECS docs.

Author: colleenmcginnis

Diff for: docs/release-notes/breaking-changes.md

@@ -92,4 +92,4 @@ We have some breaking changes in this release. We wanted to do these changes pri
 * For better naming we replaced the Elastic.Apm.All packages with Elastic.Apm.NetCoreAll. For more information, check [#371](https://github.com/elastic/apm-agent-dotnet/pull/371).
 * Based on feedback we also renamed the UseElasticApm() method in the Elastic.Apm.NetCoreAll package to UseAllElasticApm - this method turns on every component of the Agent for ASP.NET Core. For more information, check [#371](https://github.com/elastic/apm-agent-dotnet/pull/371).
 * Our logger abstraction, specifically the IApmLogger interface changed. For more information, check [#389](https://github.com/elastic/apm-agent-dotnet/pull/389).
-* To follow the [Elastic Common Schema (ECS)][Elastic Common Schema (ECS)](ecs://docs/reference/index.md)), we renamed our Tags properties to Labels. For more information, check [#416](https://github.com/elastic/apm-agent-dotnet/pull/416).
+* To follow the [Elastic Common Schema (ECS)][Elastic Common Schema (ECS)](ecs://reference/index.md)), we renamed our Tags properties to Labels. For more information, check [#416](https://github.com/elastic/apm-agent-dotnet/pull/416).