docs: Add starlark directive to code snippet (#1170)

A very minor cleaup change.

gazelle/README.md

diff --git a/gazelle/README.md b/gazelle/README.md
index e9a8052..2cd3876 100644
--- a/gazelle/README.md
+++ b/gazelle/README.md
@@ -93,7 +93,7 @@
 with the rules_python extension included. This typically goes in your root
 `/BUILD.bazel` file:
 
-```
+```starlark
 load("@bazel_gazelle//:def.bzl", "gazelle")
 load("@rules_python_gazelle_plugin//:def.bzl", "GAZELLE_PYTHON_RUNTIME_DEPS")