docs: avoid users on 3.10 by accident (#653)

README.md

diff --git a/README.md b/README.md
index 5a22e8a..ac57342 100644
--- a/README.md
+++ b/README.md
@@ -60,12 +60,13 @@
 load("@rules_python//python:repositories.bzl", "python_register_toolchains")
 
 python_register_toolchains(
-    name = "python310",
+    name = "python3_9",
     # Available versions are listed in @rules_python//python:versions.bzl.
-    python_version = "3.10",
+    # We recommend using the same version your team is already standardized on.
+    python_version = "3.9",
 )
 
-load("@python310_resolved_interpreter//:defs.bzl", "interpreter")
+load("@python3_9_resolved_interpreter//:defs.bzl", "interpreter")
 
 load("@rules_python//python:pip.bzl", "pip_parse")