appropriately escape build_test code snippet

docs/build_test_doc.md

@@ -15,6 +15,7 @@ support would be welcomed.
 
 Typical usage:
 
+```
   load("@bazel_skylib//rules:build_test.bzl", "build_test")
   build_test(
       name = "my_build_test",
@@ -22,6 +23,7 @@ Typical usage:
           "//some/package:rule",
       ],
   )
+```
 
 
 ### Parameters

rules/build_test.bzl

@@ -28,6 +28,7 @@ def build_test(name, targets, **kwargs):
 
     Typical usage:
 
+    ```
       load("@bazel_skylib//rules:build_test.bzl", "build_test")
       build_test(
           name = "my_build_test",
@@ -35,6 +36,7 @@ def build_test(name, targets, **kwargs):
               "//some/package:rule",
           ],
       )
+    ```
 
     Args:
       name: The name of the test rule.