Fix typo in usage comment
Fixes
RELNOTES: None
@@ -76,7 +76,7 @@ Example usage:
```python
# In your BUILD file...
-load("@bazel_tools//tools/python/toolchain.bzl", "py_runtime_pair")
+load("@bazel_tools//tools/python:toolchain.bzl", "py_runtime_pair")
py_runtime(
name = "my_py2_runtime",
