Fix broken link in Linkerd2 BUILD.md
Fixes a broken link to the CLI documentation in the "Generating CLI Docs" section of the Linkerd2 Development Guide.
@@ -431,6 +431,6 @@ build_architecture
# Generating CLI docs
-The [documentation](https://linkerd.io/2/reference/cli/) for the CLI
+The [documentation](https://linkerd.io/2/cli/) for the CLI
tool is partially generated from YAML. This can be generated by running the
`linkerd doc` command.
