backwards-compatibility: fix broken link

The breaking-changes-guide has moved to bazelbuild/bazel-website
which is served from bazel.build, rather than docs.bazel.build.
So we cannot use a relative link. Fix this.

Change-Id: Iee86d2c5b3d74bdf34a78fb8449c4d2c9d73c7f1
PiperOrigin-RevId: 253770406

site/docs/backward-compatibility.md

@@ -28,7 +28,7 @@ incompatible and require some changes from Bazel users.
 
 ### How to follow this policy
 
-* [For contributors - best practices for incompatible changes](../../breaking-changes-guide.html)
+* [For contributors - best practices for incompatible changes](https://bazel.build/breaking-changes-guide.html)
 * <a href='https://github.com/bazelbuild/continuous-integration/tree/master/docs/release-playbook.%6D%64'>For release managers - how to update issue labels and release</a>